This is implemented in the plugins and is not used in the base assign class.
This was also using a constant from the assignsubmission_file plugin which is
bad form.
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
This makes it consistent with the built in filtering in flexible_table.
Fix additional issue with SQL for grading table when there are no enrolled users
Change SQL for mod_assign grading table to use get_in_or_equal
As Suggested by Andrew Nicols in tracker
