Merge Policy Advisory Groups and Policy Teams

They are two formats which are very similar but neither of them quite do what is needed. Ideally they would be merged into a very generic group model but this is already taken by groups within organisations. Instead merge them into PolicyGroup, which helpfully they were both already subclassing. They are however, required to appear on the front end as just Group.

I have made a data migration to remove the old url structures from the search index and add the new objects. However, I would appreciate feedback on if that is either the correct way to make this work or if it is the preferred way.

I will create a separate pull request against router data to handle the redirection from the old paths to the new ones.

https://www.pivotaltracker.com/story/show/67300906