Fix #4913: Invalid replacements created if an error is raised during replacement

Perform the replacement in a before_create callback so that it runs in a
transaction and if it fails, the transaction will rollback and the
replacement record won't be created.

Doing the replacement in a transaction isn't great because, for one
thing, it could hold the transaction open a long time, which isn't good
for the database. And two, if the transaction rolls back, the database
changes will be undone, but if the replacement file has already been saved
to disk, then it won't be undone, which could result in a dangling file.

app/models/post_replacement.rb

@@ -3,9 +3,11 @@
 class PostReplacement < ApplicationRecord
   belongs_to :post
   belongs_to :creator, class_name: "User"
-  before_validation :initialize_fields, on: :create
-  attr_accessor :replacement_file, :final_source, :tags
 
+  before_validation :initialize_fields, on: :create
+  before_create :process!
+
+  attr_accessor :replacement_file, :final_source, :tags
   attribute :replacement_url, default: ""
 
   def initialize_fields