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.