Also, convert the user tickets template file to use ORM

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

Fixes #1660

  * Add trashcan icon for newly-added actions
  * Categorize filter actions
  * Use imperative phrases for action descriptions
  * Drop check boxes from simple actions (like reject ticket)
  * Hide empty forms on new ticket pages
  * Do not store config for nondata fields for actions
  * Implement a multi-use feature for actions, which will allow using a
    action more than once (for instance, multiple email sends)
  * Filter actions are sortable
  * Send email has from address configurable
  * %{user} token is valid as a recipient

This patch rebases filters into a row-based layout and redesigns the filter
apply method to be more extensible. It also redesigns the UI to be more
dynamic and to allow for actions to be added without database modification
and actions can also have complex configurations.

And properly escape and display form title and instructions

Help topics can now specify one or more additional forms to be included on
the help topic and can also specify the sort order of those forms.
Furthermore, individual fields can be disabled per help topic, so that
unnecessary fields can be omitted when necessary, per help topic.

The disabled flag is recorded along side the field data so that the field
will not be accidentally added to the form later automatically. There is no
interface in this commit to enable a field which was disabled by the help
topic when ticket was created.

  * Fix missing attachments join for Category (fixes #1654)
  * Fix crash on user account registration
  * Fix save button on user account (fixes #1655) — regression from bebc2d79

  * Fix translation of pages
  * Localize client thank-you page on new ticket
  * Fix adding new category
  * Fix crash on staff login when upgrading from < 1.9
  * Don't show self locks on ticket queue pages

Show the form title on edit instead of "Custom Form" - which can be
misleading since the user might be editing a built-in form.

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.