Don't migrate attachments as part of the 1.6* major upgrade. Wait unit
transitioning to schema signature dd0022fb, where the new table for
file_chunks is created.

Incremental developers will already have the columns dropped from the
%ticket_attachments table and will receive an error for the cleanup script,
but the upgrader is configured to ignore errors from the cleanup scripts.

- Move access to it's own table.
- Refactor what department membership means in relation to group access

* 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)

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.