Minor rearrangement of the view mixins

... and some minor tidying-up; removing unused imports, etc.
Reuse popit_unwrap_pagination in candidates_create_popit
Don't insist the party's in PartyData before making a membership of it

This check isn't particularly helpful, since creating the membership
should fail if the person it refers to doesn't exist anyway, and it
creates problems when initially creating unknown parties via
candidates_create_popit.
Re-raise any exception, rather than wrapping it in CommandError

Not having the traceback turns out to be frequently annoying.
Make the request optional when creating change metadata

We'd like to be able to use get_change_metadata even from helper
scripts, where there's no request.
Make create_person and update_person return the person ID

Both currently return None, so they might as well consistently return
the person ID, which is actually useful in the case of create_person.
Make sure each person has an inital entry in versions

We do this by reusing create_person from update.py to do the creation of
people and their memberships in candidates_create_popit.
Commit all Electoral Commission sourced party data

It was a mistake not committing this early - thanks to Sym Roe for
spotting this problem.
Don't include image references in the dumped party JSON
Get the maximum 200 parties per page from PopIt
Strip any trailing whitespace when dumping parties to JSON
Merge pull request #6 from mhl/further-party-changes

Further party changes