Added new notification framework

It is now possible to add notifications to
users and notifications will be shown in user's
profile page. There is also an indicator in the top
bar if there are new notifications.
Notification framework

Added also the new files which were forgotten to add to the previous commit.
Merge remote-tracking branch 'upstream/master' into notification_framework
Modifications to notification framework

Notifications are now bound to specific course instances.
Also the notification menu is now a bit different, it shows
a pop-up menu for selecting the course.
Merge remote-tracking branch 'upstream/master' into notification_framework
Merge pull request #137 from ttsirkia/master

Notification framework
Student is now notified if assistant gives them feedback on their submission.
Finishing documentation.
Merge pull request #138 from LassiHaaranen/assistant_feedback

Users are notified after assistant feedback
Merge remote-tracking branch 'upstream/master'