Fixed Ticket #492

I have fixed the problem with the application sending an incorrect
warning for removing a group from an event. This happens when the
event is not assigned to all the groups in the course.

This has been fixed and tested. The testing I have done includes:
- saving an event without changing the groups - no warning
- adding additional groups - no warning
- removing previously added groups - warning
fixed manual releasing mixed evaluation comments

The comment release status on the student view is using the
comment_release status attribute for evaluation_mixevals, which
is currently no longer in use - due to the implementation of the
ability to release individual comments

Tests done:
- Mixed
	- manually releasing all comments
	- manually releasing selected comments
	- not releasing any comments
	- auto-releasing all comments
	- auto-releasing comments / unrelease selected comments
- Rubrics - manual and auto release comments (all/selected)
- Simple - manual and auto release comments (all/selected)

No change is needed for Rubrics and Simple because the comment_release
status in their respective evaluation object is still in use. I've also
checked that releasing grades is still functional. Since releasing
grades is all or nothing, we do not need to change the computation
of the grade release boolean to array_sum.