Api links

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 https://github.com/mysociety/popit/pull/301 to work correctly.

Fixes https://github.com/mysociety/popit/issues/268
Fixes https://github.com/mysociety/popit/issues/290