comments: allow admins to remove comment votes (fix #4640)

Allow admins to remove comment votes by other users. This is done by
clicking the comment score to get to the comment vote list, then
clicking the Remove button on every vote.

config/routes.rb

@@ -81,7 +81,7 @@ Rails.application.routes.draw do
       post :approve
     end
   end
-  resources :comment_votes, only: [:index]
+  resources :comment_votes, only: [:index, :destroy]
   resources :comments do
     resource :votes, controller: "comment_votes", only: [:create, :destroy], as: "comment_votes"
     collection do