Add policy advisory groups

Policy advisory groups and policy teams are really the same thing -
these migrations reorganise them into a classical inheritance model
based on a single table like the way different types of documents work.

Policy advisory groups have an additional summary and support
attachments. Supporting attachments required adding functionality to our
govspeak helper, which is a mess.
Unify policy team and policy advisory group styles

Mangling block-$x here was a pain, so @edds and I refactored the two
views to make them basically the same, using `.content` and `.sidebar`
to make the styles more obvious.
Small changes to address feedback

Thanks @h-lame!

- Refactored policy advisory group picker to be more clear
- Changed migration to just a `change` since it was cleanly reversible
- Made foreign key relationship between policy_advisory_groups and
  policies explicit
- Avoided in-memory sorting by deferring it to SQL
Merge pull request #146 from alphagov/policy-advisory-groups

Policy advisory groups