Add the ability of revoking previous ticket state tracking events when new
events are logged for the same ticket. This will allow, for instance, the
ability to revert the 'closed' state of a ticket when the ticket is
reopened.

For statistics tracking, a user could configure whether or not the events
should be counted for each event tracked or just the non-annulled events.
For instance, if a ticket is closed and reopened several times, only the
very last closed event should count toward the statistics for the ticket.
Therefore, when a ticket is reopened, previous closed events should be
marked as annulled.

And correct several undefined function errors from several source files. So
while function names in PHP are considered case-insensitive, it still makes
sense to use consistent camel casing for both defining and calling methods.
The lint test searches the code base for method calls, and then searches the
code base again looking for a function definition matching the name of the
function invoked. It's not failsafe, because it doesn't detect the class
from which the method should belong, so it's likely to have false negatives.
Furthermore, it won't work well for PHP 5 where several classes are built
into PHP (and aren't searchable in the osTicket code base).

Remove the include/staff/api.inc.php as it no longer appears to be used (and
contains references to undefined methods).

changing ticket create/status bottons on client landing page, redoing navbar shadow with css, adding kb icons

Replace the ticket_{message,response,note} tables with a single
ticket_thread table that will contain data for all three current message
types. This simplifies much of the ticket thread code and paves the way for
other types of messages in the future.

Track assigned department, team, staff, and help topic when the ticket event
occurs. This will greatly help in correlation of various reports and queries

Also start plowing the way toward incremental database updates using a
patching technique. The hash of the main install SQL script for a respective
database will be used to track the signature of the database currently. The
signature will be stored in %config::schema_signature, and the main.inc.php
script can simply check the value in the schema against the value known to
the source code to be the signature the code expects. Should the two
signatures differ, patches in the setup/inc/sql/patches folder should exist
and be executed to assist in incrementally upgrading the database to the
new-current schema version.

Adds a link to the scp/tickets.php page (Tickets) allowing for the export of
the data to CSV file. This is preliminary and still needs a few items:
  * An icon + button CSS definition
  * Support for alternate columns (staff assigned, etc.)

Also add beginnings of complex export support (PDF, JSON  and other
formats
to follow). CSV is fully supported for now.

And for brevity, add a Http::download method to allow for consistent and
browser-independent forced page content downloads

Add a %ticket_history table to track ticket states over time, correlated to
the user account changing the ticket. Then, tickets are tracked to the table
when (re)opened, closed, overdue, transferred and assigned.