Fix method signature of formfield_for_foreignkey()

Highlighted by PyCharm that it differed from the base class.
Use populate_xheaders() on all responses, just like `django.contrib.flatpages` does

Make it easier to debug the response.
Add flatpage pagetype, make django-fluent-pages usable out-of-the-box

The flatpage is based on django.contrib.flatpages and django-flatpages-tinymce,
it features a simple "content" field with a WYSIWYG editor (e.g. TinyMCE).

For sites with simple flat content, this avoids the need to configure the
django-fluent-contents module to create simple content pages.
Include the django-wysiwyg cleanup features to the flatpage

The settings are identical to the `fluent_contents.plugins.text` plugin.
Improve appearance of keywords + description fields in the admin