migrate further fields from ResultEvent to use django-popolo models

[mhl]

Of the fields in ResultEvent, we've already migrated winner to be a foreign key to Person, but there are other updates that could be made to improve data integrity:

• winner_person_name isn't needed any more - winner.name would work
• post_id and post_name can be replaced by post (a foreign key to Post) and post.label
• winner_party_id can be replaced by winner_party (a foreign key to Organization)
• The winner_party_name property method can be replaced by winner_party.name

[mhl]

These changes have been made in the DC fork, and could be backported by cherry-picking