Allow 'Standing in 2015?' to be Yes/No/Not Sure, not just Yes/No

Fixes #43
Add a FIXME suggesting some refactoring
Use party / constituency data from 2010 as defaults (if available)

This means that if you edit a person who's not yet marked as standing in
2015, but for whom there is default data from 2010, that 2010 party and
constituency will be prepopulated in the party / constituency box when
they're unhidden.