FIXED #12810: return validation error if a registered table name is used for m2m fields

Ticket: https://code.djangoproject.com/ticket/12810

Description: Throw an error with the checks framework when m2m fields are configured to use a table that is already registered.
I'll update the text to not be wrapped in parentheses, but I think I'll keep attributes on different lines to keep things consistent with the existing code base.