[1.6.x] Fixed #21428 -- editable GenericRelation regression

The GenericRelation refactoring removed GenericRelations from
model._meta.many_to_many. This had the side effect of disallowing
editable GenericRelations in ModelForms. Editable GenericRelations
aren't officially supported, but if we don't fix this we don't offer any
upgrade path for those who used the ability to set editable=True
in GenericRelation subclass.

Thanks to Trac alias joshcartme for the report and stephencmd and Loic
for working on this issue.

Backpatch of 0e079e4331a8be4dbd18d5e5776116330b0a5e61 from master.
[1.6.x] Fixed a regression caused by fix for #21428

On Python 3 sorting Fields mixed with GenericForeignKeys doesn't work
as GenericForeignKey isn't a subclass of django.db.models.fields.Field.

Refs #21428.

Backport of 326539f6a4 from master