assign_plugin_config data was only included in backups/imports if user data was included in
the backup. This patch moves it out with the standard structure of the assign backup.
This adds separate controls for the sending of notifications from the mod_assign.
The separate notifications are: student submission receipt (for students), feedback available (for students)
and submission updated (for graders). The grader notifications can be enabled for all submissions or just late ones.
Added code to reset the message providers on upgrade (only applies for a 2.3 dev version)
