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)
Also move the 'default' setting for each plugin into it's own settings file
so that it is handled properly on new installs.
(ensure the plugin is available at setting time)
Also make the file submission plugin on by default