Custom migrations are used mainly in unit test suites to dynamically

construct migrations that can be applied to an app. Currently,

EvolveAppTask accepts these and passes them to MigrationExecutor.

However, upcoming work will involve creating a MigrationExecutor

outside of the management of EvolveAppTask, but with the need to use

the migrations registered there.
Now, EvolveAppTask registers custom migrations globally when preparing

tasks, and then urnegisters them after tasks have been prepared.
This is not thread-safe, but then, evolving the database is not

something that should ever be done by more than one thread (or process,

or server) at a time anyway. Plus this is really intended for unit

testing. So it's not a concern for this purpose.

All unit tests pass.
Tested along with upcoming work for a MoveToDjangoEvolution mutation.