Api links

(I accidentally closed #22 because I had opened it against a branch that I've now deleted, oops - this replaces it)

This builds on top of #21, giving each record a url property pointing to the singular resource in the API and an html_url property pointing the record in the UI. This needs mysociety/popit#301 to work correctly.

Fixes mysociety/popit#268
Fixes mysociety/popit#290