Add generic permission model

[MrSerth]

In order to allow users granting permissions for their tasks, collections and groups to other users, we could benefit from a generic permission model.

Unitl now, a dedicated NxN relationship between tasks and users existed. While that works for one model, it is not generic and similar logic needs to be implemented for exercises and collections.

Hence, we propose to add a generic, polymorphic permission model to CodeHarbor. The following data structure might be useful as a first draft:

context_id
context_type
user_id
permission

The context_id and context_type represent the polymorphic assosication and might be used to relate to either a task, collection or group.