Upgrader revisited

Reviewed-By: Jared Hancock

The upgrader can't (and shouldn't) do everything necessary to complete an
upgrade. The adminsitrator will need to take care of a few extra tasks that
are outlined in the UPGRADING.txt file. One of the items includes removing
source code no longer used in the 1.7 codebase. A script is shipped (Unix
only) that allows an administrator to automatically clean up all old source
files that are no longer used.

* Append page url to error alerts - useful in determining which page triggered the error.

Allow to upgrader to roll through tasks during upgrade without bread (trusting the tasks to break)

cron.php - 500 Internal Server Error

Reviewed By: Peter 07/01/12

Fix PHP crash when system is offline due to pending upgrade

Reviewed By: Peter 07/01/12

Add ticket class  (just incase the parent doesn't have it included)- required to fetch staff's ticket stats.

And provide a shell script to remove codebase files no longer used in
osTicket 1.7

Add a (temporary) key to %ticket_thread allowing quick lookups on the old_pk
column. Also, only fill the temporary table used for linking responses to
the migrated messages with messages.

Remove clobber between queue() method and queue instance property (which is
the queue list). Also properly limit the number of attachments queued for
migration as indicated by the initial call to the queueAttachments() method.
Lastly, fix an issue where the first $limit items of the database cannot be
found on or read from the disk. Recurse through the queueAttachments()
method until the entire database is scanned or $limit items are placed in
the queue.

This overcomes the eventual limit of and database to support queries of a
finite length. We now split the file contents into 100k chunks and append
the chunks to the database one chunk at a time.

Migrate the PHP session from disk to database live. To pull this off, the
session contents are written to database under the current session id. When
the `ostversion` column is dropped from the %config table, the system will
automatically switch to database-backed sessions in osTicket 1.7 mode.

This is sort-of hacked together by carefully calling a instance method of
the osTicketSession class statically, and modify the instance method to
support static invocation.

When the `ostversion` column is removed from the %config table, the system
will immediately switch to database-backed sessions. Because the
administrator upgrading the system originally logged in with a file-based
session, it must be migrated to the database before removing the column.