Remove the local polymorphic admin, it's part of django-polymorphic 0.3 now.

As django-polymorphic 0.3 is released with our polymorphic admin code,
the local copy can be removed.

Also marking the app as beta now, as the polymorphic admin API
is now released and accepted as is.
Using django-tag-parser, the tag parsing has been to that app.

This app and the django-fluent-* apps all share the same tag parsing
logic. Now no app holds a local copy anymore.
Remove utils package as it has become obsolete now.

The admin+tag parsing utils have been migrated to other apps.
The JsonResponse turns out to be unused.
Cleanup and modernize setup.py and bump version numbers