votes: allow admins to remove post votes.

Allow admins to remove votes on posts. This is for fixing vote abuse.

Votes can be removed by going to the vote list on the /post_votes page,
or by clicking on a post's score, then using the "Remove" option in the
"..." dropdown menu next to the vote.

Votes are soft-deleted - they're marked as deleted in the database, but
not fully deleted. Removed votes are only visible to admins, not to
regular users. When a vote is removed by an admin, it leaves a mod
action.

Technically it's possible to undelete votes, but there's no UI for it.

app/models/post.rb

@@ -40,7 +40,7 @@ class Post < ApplicationRecord
   has_one :upload, :dependent => :destroy
   has_one :artist_commentary, :dependent => :destroy
   has_one :pixiv_ugoira_frame_data, class_name: "PixivUgoiraFrameData", foreign_key: :md5, primary_key: :md5
-  has_one :vote_by_current_user, -> { where(user_id: CurrentUser.id) }, class_name: "PostVote" # XXX using current user here is wrong
+  has_one :vote_by_current_user, -> { active.where(user_id: CurrentUser.id) }, class_name: "PostVote" # XXX using current user here is wrong
   has_many :flags, :class_name => "PostFlag", :dependent => :destroy
   has_many :appeals, :class_name => "PostAppeal", :dependent => :destroy
   has_many :votes, :class_name => "PostVote", :dependent => :destroy
@@ -701,18 +701,10 @@ class Post < ApplicationRecord
       return unless Pundit.policy!(voter, PostVote).create?
 
       with_lock do
-        votes.destroy_by(user: voter)
-        votes.create!(user: voter, score: score)
+        votes.create!(user: voter, score: score) unless votes.active.exists?(user: voter, score: score)
         reload # PostVote.create modifies our score. Reload to get the new score.
       end
     end
-
-    def unvote!(voter)
-      return unless Pundit.policy!(voter, PostVote).create?
-
-      votes.destroy_by(user: voter)
-      reload
-    end
   end
 
   module ParentMethods