db: do case-insensitive explicit sorting of RepoGroup names

This does not change the implicit sorting enabled via __mapper_args__,
which is a bad idea anyway (and now deprecated), and will have to be
dealt with in a later changeset.

The use of __mapper_args__ implicitly adds sorting to every query of
RepoGroup objects throughout the code (including implicit queries via
relationships). For the relationships, __mapper_args can be replaced
with "order_by" on each individual relationship, and it's reasonably
straight-forward to identify every RepoGroup query throughout the code,
and add explicit sorting. But we don't really need that sorting most
of the time, so a better way forward may be to identify all the places
that actually needs the sorting, make it explicit there, and then kill
the __mapper_args__. (Anyway, future work.)