Separation of Group entity and its Repository

[jwgmeligmeyling]

Group and Repository should be separated. This can be useful for the following things:

• It is confusing to speak of a Group when you want a Repository. Things as inline Comments or Commits should simply not be attached to a Group from a ER point of view.
• Separating Group and Repository cleans up the way for having multiple repositories per course, or supporting private repositories (Support private repositories #252)