Introduce the concept of a feature set, which can be
registered to an entrypoint.
Rework all existing core elements into a 'core' feature
set.
Remove the ability to add in random django apps, and drop
the ablity for plugins to optionally be able to great
new DB models.
Change-Id: Idc5c3bf3facc44bb615fa4006d417d6f48a16ddc
0eaac89b38