Try to avoid the new migration requirement

[nickmoreton]

In a test on an existing site implementing a ModelViewsSet to replace wagtail-orderable resulted in a new migration.

Can this be avoided?

https://github.com/torchbox/rca-wagtail-2019/pull/1100/files#diff-86011f9da927c685a1c94f9837d9e496f4d06fc6314d83f18762586c5b4c949a