Fixed button colors
The exercise button was incorrectly green if
the exercise was worth of zero points and the
user had not submitted the exercise.
Merge pull request #72 from ttsirkia/master

Fixed button colors
The if statement in the old clean caused DoesNotExist error if course_module or category was not set. That way, the model forms in Django admin could not display the validation error correctly in the invalid form.
- Edited Submission.check_user_permission to also allow users with User.is_staff to access the submission.
- Edited some comments in Submission.check_user_permission
- Denied access to view_submission from everyone else but the submitters of the submission because the view would make no sense to staff-like users.
	- This also fixes the bug where the assignment of the index caused error in case anyone but the submitters of the submission tried to access this view.
	- Added a comment about that to the code because check_user_permission might seem the right way to go
Merge pull request #73 from Rubinous/view_submission_of_someone_else_bug_fix

Bug fix - Trying to view someone else's submission page caused HTTP 500
Merge pull request #75 from Rubinous/fixed_LearningObject_clean

The if statement in the old clean caused DoesNotExist error if course_mo...