Adds Wagtail Formpages to the mix, too.

[bahoo]

Description of change

I had a legacy form page which inherits from Wagtail's own FormMixin, rather than via CodeRed's models. This change simply includes those pages in the admin. My form submission results were inaccessible, otherwise.

Tests

1. Create a page model which inherits from Wagtail's forms, e.g.

from wagtail.contrib.forms.models import AbstractEmailForm
from wagtail.fields import RichTextField

class FormPage(AbstractEmailForm):
   intro = RichTextField(blank=True)

2. Make migrations, and migrate.
3. Create a new page for this FormPage in the Wagtail admin.
4. Verify that your newly created FormPage is now visible under the Forms tab in the admin.

[vsalvino]

Good catch, thank you for contributing!