core/models: add RepositoryRelation associated to repositories

[hertzog@debian.org:
- rename RepositoryRelation attributes: initial_repository ->
  target_repository, value -> name
- also add related_name to target_repository attribute, and test it
- cleanup a test where a value was needlessly stored]

Signed-off-by: Raphaël Hertzog <hertzog@debian.org>