* DEPLOYMENT NOTE: run "/fix_datastore" as "admin" after deployment
* removed userId from Vote
* added Backer to Vote model
* refactored VoteForProposal to support changes
* changed Proposal template to show Backer's "username" instead of "email"
* implemented "fix_datastore" deferred jobs to updated existing Vote models (create Backer based on userId, remove userId for Votes that have Backer as not None)