Django's standard MigrationExecutor only worked with on-disk

migrations, which was difficult for testing. We worked around the

limitations by patching some of the generated migration graph, but this

isn't always feasible, and isn't generic enough to use outside of

limited unit tests.
This change introduces a subclass of MigrationExecutor and

MigrationLoader that supports custom migrations, which will not only

be useful for tests but for the upcoming Evolver support.
This subclass also consolidates some important tests (handling Django

version differences in the process) and introduces new signals that will

be emitted during the migration application process.

Unit tests pass on all supported versions of Django.