post_vote.rb: determine vote magnitude from voter, not CurrentUser.

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=`.

app/models/post.rb

@@ -1063,7 +1063,7 @@ class Post < ActiveRecord::Base
       !PostVote.exists?(:user_id => user.id, :post_id => id)
     end
 
-    def vote!(score, voter = CurrentUser.user)
+    def vote!(vote, voter = CurrentUser.user)
       unless voter.is_voter?
         raise PostVote::Error.new("You do not have permission to vote")
       end
@@ -1072,7 +1072,7 @@ class Post < ActiveRecord::Base
         raise PostVote::Error.new("You have already voted for this post")
       end
 
-      votes.create!(user: voter, score: vote)
+      votes.create!(user: voter, vote: vote)
       reload # PostVote.create modifies our score. Reload to get the new score.
     end