Bug: when moving favorites, the parent is given an upvote for each gold+
favoriter, but the magnitude of these upvotes was based on the
CurrentUser, rather than the upvoter. This meant that upvotes for
supervoter favorites weren't given correctly.
To fix this, `magnitude` is changed to use the voting `user` instead of
CurrentUser.
New problem: when setting the score, `score=` calls `magnitude`, but
`user` isn't initialized yet. So we also refactor so that
1. `initialize_attributes` initializes `user` before setting `score`
2. the vote direction is given by `vote`, so it's separate from `score`
3. updating the score on the post happens in a callback instead of
directly in `score=`.
