Review "Activity"
The concept of Activites should be reviewed, so it is used consequently to trace relevant information. Also, it might be possible to use django-reversion revisions and tags instead.
As we are discussing to turn various parts of AlekSIS into a generic calendar interface, it might be considered that activities should be events in the calendar as well, and that we might actually not want to have real model instances for activities, but rather an API to ask apps for activities on models.