posts: rework post deletion to use dialog box.

Rework post deletion from using a separate page to using a dialog box,
like flagging.

* Add `DELETE /posts/:id` endpoint.
* Remove `POST /moderator/post/posts/:id/delete` endpoint.

app/controllers/moderator/post/posts_controller.rb

@@ -4,18 +4,6 @@ module Moderator
       skip_before_action :api_check
       respond_to :html, :json, :xml, :js
 
-      def confirm_delete
-        @post = ::Post.find(params[:id])
-      end
-
-      def delete
-        @post = authorize ::Post.find(params[:id])
-        if params[:commit] == "Delete"
-          @post.delete!(params[:reason], :move_favorites => params[:move_favorites].present?)
-        end
-        redirect_to(post_path(@post))
-      end
-
       def confirm_move_favorites
         @post = ::Post.find(params[:id])
       end