Rewrite media_data handling to use relationships

Rewrite media_data handling to use relationships

Instead of doing query by hand, use the relationships on
the models to find the media_data. Is is made possible by
the BACKREF_NAME in each models.py, which lets us know the
local attr to ask for.

Also initialize the relationship attribute on new
media_data instead of the media_id. Also do not add it to
the session. This gives us:
- This automatically initializes the other side of the
  relationship, which will allow later acces via that way.
- If the media_data is too early in the session, when the
  (new) media_entry is not yet in there, this could get
  conflicts. Avoid those by not adding to session.
- Uses cascading to commit media_data together with the
  media_entry.