Enforce inheriting the ContentItemForm for plugin forms.
Enforce inheriting the ContentItemForm for plugin forms.

Breaks further down otherwise, which causes confusing errors.

ixc/django-fluent-contents,edoburu/django-fluent-contents,django-fluent/django-fluent-contents,edoburu/django-fluent-contents,ixc/django-fluent-contents,pombredanne/django-fluent-contents,jpotterm/django-fluent-contents,pombredanne/django-fluent-contents,django-fluent/django-fluent-contents,jpotterm/django-fluent-contents,django-fluent/django-fluent-contents,edoburu/django-fluent-contents,pombredanne/django-fluent-contents,jpotterm/django-fluent-contents,ixc/django-fluent-contents