Fix #4969: Tag changes made by replacements wipe out edits done at the same time.

Lock the post during replacement to ensure we have the latest version of
the tags and to ensure nobody else can modify the post until after the
replacement is finished.

app/models/post_replacement.rb

@@ -12,8 +12,6 @@ class PostReplacement < ApplicationRecord
 
   def initialize_fields
     self.original_url = post.source
-    self.tags = "#{post.tag_string} #{tags}"
-
     self.old_file_ext = post.file_ext
     self.old_file_size = post.file_size
     self.old_image_width = post.image_width