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/controllers/post_replacements_controller.rb

@@ -9,11 +9,11 @@ class PostReplacementsController < ApplicationController
   end
 
   def create
-    @post = authorize Post.find(params[:post_id]), policy_class: PostReplacementPolicy
-    @post_replacement = @post.replace!(permitted_attributes(PostReplacement))
+    @post_replacement = authorize PostReplacement.new(creator: CurrentUser.user, post_id: params[:post_id], **permitted_attributes(PostReplacement))
+    @post_replacement.save
+    @post_replacement.process!
 
-    flash[:notice] = "Post replaced"
-    respond_with(@post_replacement, location: @post)
+    respond_with(@post_replacement, location: @post_replacement.post, notice: "Post replaced")
   end
 
   def update