post replacements: refactor and fix tests.

* Move replacement tests from test/unit/upload_service_test.rb to
  test/functional/post_replacement_controller_test.rb
* Move UploadService::Replacer to PostReplacementProcessor.
* Fix a minor bug where if you used the API to replace a post with a file,
  the replacement would fail unless you passed an empty string for the
  replacement_url.

app/models/post_replacement.rb

@@ -6,8 +6,9 @@ class PostReplacement < ApplicationRecord
   before_validation :initialize_fields, on: :create
   attr_accessor :replacement_file, :final_source, :tags
 
+  attribute :replacement_url, default: ""
+
   def initialize_fields
-    self.creator = CurrentUser.user
     self.original_url = post.source
     self.tags = "#{post.tag_string} #{tags}"
 
@@ -27,6 +28,10 @@ class PostReplacement < ApplicationRecord
     end
   end
 
+  def process!
+    PostReplacementProcessor.new(post: post, replacement: self).process!
+  end
+
   def suggested_tags_for_removal
     tags = post.tag_array.select do |tag|
       Danbooru.config.post_replacement_tag_removals.any? do |pattern|