Improve information in recent activity block

The recent activity block only displays information about the
groupdistributio if the rating period is currently running.
It furthermore notices the user if the rating period is running.
Fix missing $a variable

Readded the deleted $a variable.
Fix issue #1 (database error)

Moodle expects a column named course in the groupdistribution table.
Renamed the courseid clumn to course and changed all references in
the code.
The groupdistribution plugin has to reinstalled and cannot be
upgraded. There is a notification about this in upgrade.php.