posts: allow toggling between upvotes and downvotes.

Like 9efb374ae, allow users to toggle between upvoting and downvoting a
post without raising an error or having to manually remove the vote
first. If you upvote a post, then downvote it, the upvote is
automatically removed and replaced by the downvote.

Other changes:

* Tagging a post with `upvote:self` or `downvote:self` is now silently
  ignored when the user doesn't have permission to vote, instead of
  raising an error.
* Undoing a vote that doesn't exist now does nothing instead of
  returning an error. This can happen if you open the same post in two
  tabs, undo the vote in tab 1, then try to undo the vote again in tab 2.

Changes to the /post_votes API:

* `POST /post_votes` and `DELETE /post_votes` now return a post vote
  instead of a post.
* The `score` param in `POST /post_votes` is now 1 or -1, not `up` or
  `down`.

app/models/post.rb

@@ -633,7 +633,8 @@ class Post < ApplicationRecord
           remove_favorite(CurrentUser.user)
 
         when /^(up|down)vote:(.+)$/i
-          vote!($1)
+          score = ($1 == "up" ? 1 : -1)
+          vote!(score, CurrentUser.user)
 
         when /^status:active$/i
           raise User::PrivilegeError unless CurrentUser.is_approver?
@@ -779,8 +780,7 @@ class Post < ApplicationRecord
 
     def add_favorite!(user)
       Favorite.add(post: self, user: user)
-      vote!("up", user) if Pundit.policy!(user, PostVote).create?
-    rescue PostVote::Error
+      vote!(1, user)
     end
 
     def delete_user_from_fav_string(user_id)
@@ -789,8 +789,7 @@ class Post < ApplicationRecord
 
     def remove_favorite!(user)
       Favorite.remove(post: self, user: user)
-      unvote!(user) if Pundit.policy!(user, PostVote).create?
-    rescue PostVote::Error
+      unvote!(user)
     end
 
     def remove_favorite(user)
@@ -871,30 +870,22 @@ class Post < ApplicationRecord
   end
 
   module VoteMethods
-    def can_be_voted_by?(user)
-      !PostVote.exists?(:user_id => user.id, :post_id => id)
+    def vote!(score, voter)
+      # Ignore vote if user doesn't have permission to vote.
+      return unless Pundit.policy!(voter, PostVote).create?
+
+      with_lock do
+        votes.destroy_by(user: voter)
+        votes.create!(user: voter, score: score)
+        reload # PostVote.create modifies our score. Reload to get the new score.
+      end
     end
 
-    def vote!(vote, voter = CurrentUser.user)
-      unless Pundit.policy!(voter, PostVote).create?
-        raise PostVote::Error.new("You do not have permission to vote")
-      end
+    def unvote!(voter)
+      return unless Pundit.policy!(voter, PostVote).create?
 
-      unless can_be_voted_by?(voter)
-        raise PostVote::Error.new("You have already voted for this post")
-      end
-
-      votes.create!(user: voter, vote: vote)
-      reload # PostVote.create modifies our score. Reload to get the new score.
-    end
-
-    def unvote!(voter = CurrentUser.user)
-      if can_be_voted_by?(voter)
-        raise PostVote::Error.new("You have not voted for this post")
-      else
-        votes.where(user: voter).destroy_all
-        reload
-      end
+      votes.destroy_by(user: voter)
+      reload
     end
   end