Scheduling a task at the end of each origin-head and revision-metadata task
was way too slow (approx. 5 seconds, see T1492).
This Diff adds a new indexer that calls the three other indexers without
going through the scheduler, which is a lot faster and requires less code.
Further Diffs will clean up the code that is now useless.
I think it will be better to create a swh.auth.django.models module and put that model in it.
In a similar manner, authentication backends should be put in a swh.auth.django.backends (in an other diff of course).