Also log client edits as an event

This patch removes several of the internal notes posted to a ticket for
various events and replaces them with events added to a threads event chain.
The emails are still sent out as they previously were, but internal notes
are only added to the thread if an agent actually specified comments.

Move collaboration implementation to thread level

This will allow for any threadable object to render the thread.

This allows for specifying that attachments from something like a new
message should be brokered with autoresponses and alerts.

Add in the ability for an agent to resend a response. Optionally editing the
response before sending it, and setting the signature as is possible with
the usual responses.

When the response is resent, the edited version is marked as GUARDED, and
subsequent edits will result in new links in the history chain. That is,
when a response is edited and resent by an agent, that response will remain
in the history chain.

The new role would like be assigned to power users and would all such users
access to edit any post by any user.

Agents can always edit their own posts, and department managers can edit all
posts while the ticket is in the department they manage.

Threads can be edited by marking the original as hidden and setting it's PID to
the id of the new entry. The new entry has cloned data from the original and
sets the `updated` timestamp to reflect the time of last edit. An edited flag
is added to the new entry to reflect its origin.

This patch suggests that agents can edit their own posts, department managers
can edit posts while the ticket is in their department, and that help desk
admins can edit anything.

If a post is edited more than once, only the most recent edit is kept.

Several mail clients do not return the Message-ID header in a In-Reply-To or
References header as they should. The assumption by such mail clients is
that the email be threaded based on the Subject header. However, osTicket
does not require the ticket number to be placed in the Subject header and so
has trouble threading email from such systems. osTicket embeds a copy of the
Message-ID header (or compatible version) in the body of the message in
hopes part of the message will be returned.

Many mail clients (such as osTicket) strip and clean the HTML when
processing HTML email. Previously, the message-id token was embedded in a
@data-mid attribute, which was likely stripped before the HTML email would
be returned back to osTicket.

This patch suggests that the token be placed in a @class attribute, which
has a much better chance of returning to osTicket.

The %ticket_email_info table needs an index on thread_id. This is done in
the `develop-next` branch; however, this patch helps alleviate performance
issues on the main branch until the `develop-next` branch is collapsed for
main line development.

If the system receives an email by a collaborator which has not yet been
added to the ticket (a friend of a friend — that is, a collaborator forwards
an email to a third-party), a header is added to the thread body something
like:

Received From: afriendofafriend@mycompany.tld

However, if the thread body is text and the HTML ticket thread is enabled,
then the text formatting hint will be lost and the body will be assumed as
HTML deeper inside the thread entry creation process. Therefore, the
whitespace inside the resulting thread entry will be collapsed.

This patch addresses the issue by maintaining the original format hint with
the thread body.

This patch includes a slight database migration, and adjusts the
functionality of a few core components.

  * Move collaborators from the ticket to the thread.
    This concept allows collaborators on any object which has a thread,
    including tasks.

  * Add flags to the thread entry
    This will allow flagging thread entries for different purposes.
    Initially this can be used to flag the original message of a thread in
    case a ticket / thread is created without an initial message.

  * Lock becomes more of a utility
    The lock is now disconnected from the ticket and is a separate utility.
    Separately, the ticket and task objects can have a reference to a lock
    object. Furthermore, when submitting some activities to tickets, the
    lock is verified to be owned by the respective agent, and the lock code
    must match a current lock code. The code is rotated on each acquire()
    call to guard against double submissions.

  * Collaborator is an ORM model
    The TicketUser class is broken up now so that the collaborator instance
    can exist apart from a ticket. Email message ids are now generated for
    collaborators without respect for a ticket so that collaborators can be
    properly supported on any thread.

This would be required for new ticket auto-responses to tickets created
without an initial message.

Since tickets can be created without a message now, the emailing system
should also identify the thread the message is associated with, so that a
returning email can be associated with the ticket or task's thread in the
event that it was created without an initial message.

Also try harder to send a relevant In-Reply-To and References header back
to the client with the email message.

This patch suggests a change to the message-id creation process that
includes stamping the receiving user-id (staff or client) along with the
thread-id of the originating notice. This allows detection of threading if
the clinically brain-dead mail client drops all the other header detection
mechanisms, including the tag placed in the email body, on response.

This patch works for both client and agent communication.

This patch converts the central file and threading classes over to use the
ORM.

Also try harder to send a relevant In-Reply-To and References header back
to the client with the email message.

  * Fix fatal errors adding a new FAQ (fixes #1648)
  * Fix removal of attachments when deleting a thread (fixes #1650)
  * Fix crash discovering department members for alerts (fixes #1652)

This script adds a single download script, 'file.php', which provides access
to files of all types to all users. It uses a HMAC signature system with an
expires time, which allows signed URLs to be sent to external users.

This also fixes an issue with the Http::cacheable() method, where the
last-modified and Etag headers were not properly compared, which resulted in
permanent cache misses by the client.