Rename wagtail-specific app and remove django-flags duplication

[willbarton]

This PR corresponds to cfpb/django-flags#1 and closes #28.

This is the wagtail-flags side of splitting this library up into a pure-Django flags library and one that provides Wagtail support on top of that.

This is a breaking change, sort of. This PR does two major things:

• Removes all of the non-Wagtail functionality that is in Split out pure-Django feature flags from Wagtail-flags django-flags#1 and adds django-flags as a dependency
• Renames the Wagtail-specific app to wagtailflags. Django-Flags will adopt the flags app.

Any Django projects that were previously using wagtail-flags will now need to make sure they include the following apps in INSTALLED_APPS:

INSTALLED_APPS = (
    ...
    'flags',
    'wagtailflags',
    ...
)

Because Wagtail-Flags requires Django-Flags, only including flags will mean that all previous functionality continues as expected (except for Wagtail Site-based flag conditions), but the Wagtail admin UI will be gone.

That is why this will be version 3.0.0.

This PR won't pass CI until cfpb/django-flags#1 is merged.