Add an "upvote" command

Rough idea:
- User A posts an awesome shout
- User B and C reply with `^`
- A field on user A's shout is incremented and user B and C's uids are stored to a `:up` sub doc of the shout
- On interaction with the upped shout (e.g. hover) the data from `:up` is retrieved and shown in a nice way
