Group constituency offices by party on the province page.

Fiddlier than I hoped because you can't put a grouper in the context
like I tried to in c49de4083939fac172eb6d676888297d0ad20d02. It looks
like someone tried to get a patch to allow this into Django way back in
2005, but didn't write a convincing enough explanation as to why it would
be useful to interest Adrian: https://code.djangoproject.com/ticket/1461

Instead I've used more little templates to achieve a similar result.