Previously, if the quoted response mechanism was disabled, then outgoing
mail would also not include the message-id token. This breaks the
correlation of email to ticket-thread. Now, the message-id token is always
embedded in HTML emails even if the quoted-response removal system is
disabled.

(Turns out that the message-id token was always included in text bodies.)

Send an empty return-path envelope when sending out system alerts. If they
should happen to bounce for any reason, they should not return to the system
and create tickets.

If the HTML ticket thread is disabled, outgoing emails will have a text
thread body placed inside an HTML template. The template and message are
then sanitized and converted to text. However, htmLawed will munge the white
space in the message before converting to text.

This patch disables sanitizing. I think it's fair to assume that the
template and the message by the client or agent have been properly sanitized
prior to sending out the email.

Outbound message-ids are predictable, with a consistent code unique and
static to each osTicket installation. This will help detect email loops
where message is delivered back to the system in an email loop.

This approach will help detect responses to the ticket thread from mail
clients which do not include the References and In-Reply-To MIME headers.
This also allows the Subject line to continue to omit the ticket number.

This should speed up the system where several emails and alerts will be sent
through the same SMTP account in the same request.

Some emails providers (e.g gmail/google) won't deliver emails marked
bulk/autoreply to group email address.

  * Include a `bk` column to store the storage backend
  * Include a `signature` column which represents a repeatable hash of the
    file contents
  * Rename `hash` to `key` since it isn't a real hash

This is especially important if the html ticket thread is disabled. In such
a case the message body will still be HTML, but an HTML email should not be
sent out.

The detection of inline images in the mail processor causes inline images to
be removed from the attachment list, if listed there. However, the
attachments in the attachment list were not keyed by the attachment-id, so
there was no proper way to detect and remove the attachments from the list.

This patch properly keys the attachment list by the attachment file_id

If the HTML ticket thread is enabled and the message is not explicitly
declared to be text, then assume it is an HTML message.

Previous the options defined for the Mailer::send() function were confusing
when sending an In-Reply-To header

Previously, the References header emitted by the system for new thread
entry auto-responses and alerts included the message-id for which the email
was a reply in the `References` header. This patch corrects the issue by
placing the reply message-id in the In-Reply-To header, and constructs a
correct References header.

That is, if available, the References header received in the email that was
used to create the thread entry is appended to the message-id the email is
actually in reply to. This is the expected usage of the References header.

This patch changes the behavior of the mailer to support a 'text' option to
hint if the message is a text-only message. If so, no HTML processing will
be performed on the message and a text-only email (with no inline
attachments) will be emitted and sent.

Process inline attachments in thread entry and support inline images in
piped emails

Support inline images across the system, with draft support

Migrate to a single attachment table
    That way we don't need a new table for everything we need to attach an
    inline image to (like a signature, for instance)

Add richtext support for internal notes

Implement images on site pages

* Image paste in Redactor
* Make non-local images optional
* Placeholder for non-local images
* Fix local image download hover
* Don't re-attach inline images

Some mail clients and mail brokers do not properly handle quoted-printable
encoding, used in osTicket outgoing emails. Oddly, the end mail clients
render the email with trailing equal signs (=) at the end of every line,
where it was added due to QP encoding.

References:
http://www.osticket.com/forums/forum/osticket-1-7-latest-release/troubleshooting-and-problems-aa/10708-solved-microsoft-exchange-internet-mail-lines-end-with
RFC 2045 section 6.7, http://www.ietf.org/rfc/rfc2045.txt

This patch affords an administrator the ability to remove the
[#%{ticket.number}] from the email template subject line for the new ticket
autoresponse and the new message autoresponse. Previously, the ticket number
with a prefixed hash in brackets was used to identify which ticket thread an
email was in reference to.

With this patch, the email message-id (which was already kept on file) is
sent in the MIME "References" header. When a user responds to and
autoresponse email, the "References" will include this message-id in the
return email. The ticket thread is then matched up with the email based on
the message-id rather than the subject line.

Ticket numbers are still supported in the subject line, in the event that
non-compliant email clients do not properly include the References header.