Policy advisory groups

This adds [policy advisory groups](https://www.pivotaltracker.com/story/show/40179189) to the data model.

Since they're basically the same as policy teams, but with a few extra fields, I've refactored the data model to unify the two in an STI model called `PolicyGroups`.

The ticket and its behaviour is described in the `policy-advisory-groups.feature` feature file.