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.rb

@@ -842,13 +842,6 @@ class Post < ApplicationRecord
         end
       end
     end
-
-    def replace!(params)
-      replacement = replacements.create(params)
-      processor = UploadService::Replacer.new(post: self, replacement: replacement)
-      processor.process!
-      replacement
-    end
   end
 
   module VersionMethods