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.

test/factories/post_replacement.rb

@@ -3,5 +3,6 @@ FactoryBot.define do
     post factory: :post, source: FFaker::Internet.http_url
     original_url { FFaker::Internet.http_url }
     replacement_url { FFaker::Internet.http_url }
+    creator
   end
 end