Conditionally improve performance of `is_active_for_user` (#346)
Conditionally improve performance of `is_active_for_user` (#346)

`is_active_for_user` will fetch all groups for a user even if there are no group ids (meaning that the user's groups are pointless).  This commit changes that.  The final result of is_active_for_user should never change, but this improves post-cache per-request performance when checking a large number of Flag enrollments
rodgomes/django-waffle,rodgomes/django-waffle,rodgomes/django-waffle,willkg/django-waffle,willkg/django-waffle,rsalmaso/django-waffle,rsalmaso/django-waffle,rsalmaso/django-waffle,rodgomes/django-waffle,rsalmaso/django-waffle