Fixed #23938 -- Added migration support for m2m to concrete fields and vice versa

Ticket: https://code.djangoproject.com/ticket/23938
Might be a bit confusing to claim this is fixed for the vice versa case -- Yes, the auto-detector works but there'll still be an error running the migration `ValueError: Cannot alter field changem2m.Author.publishers into changem2m.Author.publishers - they are not compatible types (you cannot alter to or from M2M fields, or add or remove through= on M2M fields)` as far as I can tell.