Completely redo the way we install baselines after syncdb/migrate.
Completely redo the way we install baselines after syncdb/migrate.

Historically, Django Evolution would set up its baselines (`Version` and
`Evolution` entries) in response to a syncdb (and, in recent
development, migrate). Along with this, it would output messages saying
which apps needed to be evolved, and which baselines were being
installed. This was designed in part to help provide instructions, since
`evolve` had to be run alongside `syncdb`, and the order mattered.

With the introduction of the `Evolver`, the entire process is designed
to be done in one command (`evolve --execute`). There's no longer a need
to output any hints or to diff any signatures. The generation of
baselines can also be streamlined, and database writes optimized,
massively reducing unit test runs.

Further, we no longer need or want to do this every time we react to
this signal. If an `Evolver.evolve()` operation is running, then
baselines are already being managed, and there's no point in doing
anything at all. Instead, we only need to do this when we get these
signals outside of the `Evolver` (such as during unit tests or database
flushes).

Testing Done:
Unit tests pass on all supported versions of Django.

Reviewed at https://reviews.reviewboard.org/r/10581/

beanbaginc/django-evolution