9ae01bf2 turned off HTML balancing, however,
quoted response removal will create unbalanced HTML. That HTML will be
stored unbalanced and will require extra processing when displayed on the
ticket thread.

This patch adds balancing support using libxml if enabled, which will
balance HTML without corrupting the HTML like htmLawed will with balancing
enabled.

* Reindexing did not properly flush the last batch of items to the search
  therefore reindexing would always miss the last few items.
* Creating a new html thread entry with inline images resulted in empty
  search content
* HTML tag stripping in HtmlThreadBody::getSearchable() would result in
  missing white space between some words, resulting in poor searchable
  content

Which seems to be used by recent versions of Andriod OS. It appears to be a
synonym of `multipart/related`

Also, fix roundtrip encryption if the form name changes (which is based on a
user's session), and avoid saving a NULL value if the password is not
changed when a form is updated.

If navigating back to the ticket queue (after closing a ticket, for
instance), return to the queue previously viewed.

(Most notably is the mail from Outlook, which embeds block elements inside
of <font> elements.)

If the email address in use by the end user is not valid, display a warning
on the ticket view page. This allows an agent to easily flag potential spam
or correct a legitimate email address before sending a response and
receiving a bounce notice.

This allows detection of some incorrectly-typed email addresses before
tickets, users, and agents are created.

Preserve text formatting on the internal note posted to a new ticket by
staff if HTML thread is disabled.

This is necessary for some mail clients which indicate encoding in GB2312
but actually use GBK encoding.

If osTicket was improperly or incompletely upgraded, then some tables may
exist which should otherwise not exist. This patch assists the upgrade
process by dropping the tables which should not exist prior to adding or
renaming tables.

This correctly handles a bounce message with the following layout:

multipart/report; report-type=delivery-status
    multipart/alternative; differences=Content-Type
        text/plain; charset="us-ascii"
        text/html; charset="us-ascii"
    message/delivery-status
    message/rfc822

Fixes #1854

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.

Follow-up PR to the comment here:
https://github.com/osTicket/osTicket-1.8/commit/06d3a0f3e2a4557ca72faa916859b9315635dfe4

Copy+Pasting images from a ticket thread or from a FAQ article to the HTML
editor and submitting will result in the URL *not* being properly rewritten
for storage in the database. When images are uploaded, a relative URL is
returned from the server and embedded in the HTML editor. However, when
images from the system are copy and pasted into the editor, an absolute URL
with the http:// scheme is included. This URL was not properly handled by
the URL rewriter in Format::localizeInlineImages().

This patch addresses the issue by properly handling both relative URLs from
image uploads and absolute URLs from copy and pasting.

Fixes #1926