- Container machinery got introduced in 143ae7a4a964, at that time the
  "container" relation was mandatory (on subject) for all entity types.
- Integrity of this relation relied on the assumption that the relation
  was mandatory. (I.e. all entities mush have a link to their container
  and the only way to remove this link is to remove the container entity
  itself).
- In a88deb387b2b, this assumption got broken as "container" relation
  was made optional for SEDAArchiveUnit as a subject. From there, when
  an archive unit got unlinked from a profile (through deletion of the
  "seda_archive_unit" relation), a "container" relation remained set.
- This is problem since permission (especially on "delete" action)
  relies on the presence or absence of this relation. For instance,
  one would get an error when trying to delete an archive unit they just
  unlinked from a profile because they have no rights to delete the
  profile.

So we fix this by dropping the "container" relation when a
"seda_archive_unit" relation is deleted through a new hook.

Additional test goes in test_entities.py as other container-related
tests live there.

In migration, we drop spurious "container" relation (not sure there are
some).