FEATURE: Include a user's pending posts in the topic view

Also includes a refactor to TopicView's serializer which was not
building our attributes using serializers properly.

db/migrate/20190412161430_add_created_by_index_to_reviewables.rb

@@ -0,0 +1,5 @@
+class AddCreatedByIndexToReviewables < ActiveRecord::Migration[5.2]
+  def change
+    add_index :reviewables, [:topic_id, :status, :created_by_id]
+  end
+end