Save form_overrides into CustomModelConverter, to fix #421, in conjunction with https://github.com/coleifer/wtf-peewee/pull/20

I've taken the path of adding most of the necessary to wtf-peewee, because that's where the functionality seems to fit best (to avoid having to duplicate a big function from wtf-peewee).

This change is backwards-compatible so that old code will work with this flask-admin change, even without having to update the wtf-peewee version.

Refer https://github.com/coleifer/wtf-peewee/issues/18 and https://github.com/coleifer/wtf-peewee/pull/20 and https://github.com/mrjoes/flask-admin/issues/421