Add a page with a simple list of constituencies

Fixes #56
Add tests for the twitter_username validator
Don't allow @parliament.uk email addresses

As various people have pointed out in #63, when parliament dissolves
before the election the @parliament.uk email addresses shouldn't really
be used, so we should try to collect alternative email addresses instead
of the parliament.uk ones. This commit adds a validator that checks that
the email address isn't an @parliament.uk one.