Allow using shorthand names & __latest__ for MigrationTargets

[jasperfirecai2]

When running migrations in the commandline, it is possible to use just <app_name> <leading number> as arguments. However, when defining MigrationTargets for apply_initial_migration or apply_tested_migration, the full name is required.

Having only leading numbers be accepted would have it be more consistent with the manage.py commandline, but also wont break test cases in case a migration is renamed (e.g. from auto-generated to manually named)

__latest__ Would be a great shortcut for tests to check from <some initial state> to the newest version always.