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

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.