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.

config/routes.rb

@@ -189,7 +189,7 @@ Rails.application.routes.draw do
   end
   resources :post_regenerations, :only => [:create]
   resources :post_replacements, :only => [:index, :new, :create, :update]
-  resources :post_votes, only: [:index, :show]
+  resources :post_votes, only: [:index, :show, :create, :destroy]
 
   # XXX Use `only: []` to avoid redefining post routes defined at top of file.
   resources :posts, only: [] do