post replacements: add "Tags" field to dialog box.

* Adds a "Tags" field to the post replacement dialog box. The given tags
  are added to the post after replacement.

* Prefills the Tags field with certain tags that usually need to be
  removed after replacement: replaceme, image_sample, jpeg_artifacts, etc.

app/controllers/post_replacements_controller.rb

@@ -23,6 +23,6 @@ class PostReplacementsController < ApplicationController
 
 private
   def create_params
-    params.require(:post_replacement).permit(:replacement_url, :replacement_file, :final_source)
+    params.require(:post_replacement).permit(:replacement_url, :replacement_file, :final_source, :tags)
   end
 end