* Move migration scripted tasks to the stream folder naming them .task.php.
* Add a MigrationTask abstract class to server as the base of migration
  tasks.
* Move the migration tasks from class.upgrader to the new task files

Add forgotten abort() method of the Upgrader trampoline class, and allow
getErrors() not to crash if not upgrading

Allow `streams.cfg` to be implied as 'core', which will default to only
upgrading the core osTicket stream.

Use signature to describe snapshots rather than md5

This patch allows the upgrader to upgrade multiple database update
'streams'. The main stream is renamed to 'core' to reflect changes made
to the config class. This will significantly ease customizations
requiring database customizations, and will make plugins requiring
database changes possible.

- timezone_offset was dropped from the config table at 1.7-dpr1
- upgrader: only apply five patches in one request
- upgrader: fix readPatchInfo to work correctly
- session: support migrating from 1.6 (again)
- config: support migrating from 1.6 (fallback)
- config: no default for 'isonline' setting
- config: fix SQL whitespace issue for schema signature fallbacks
- config: hash 1.6 versions in the schema signature lookup
- upgrader: fix logging bug in attachment migration

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.