Support visions when sharing

- Add sharers to supporters on the server
- Copy all existing sharers into supporters
- Change the sharers list to a list of primary keys (to cut down on unecessary joins)
- Check for supporter primary key instead of complete model membership on the client
- Ensure user feedback after retweeting
Cache users' oauth and social id information for a very short time, so that multiple DB hits aren't made when they don't need to be