FEATURE: New 'Reviewable' model to make reviewable items generic

Includes support for flags, reviewable users and queued posts, with REST API
backwards compatibility.

Co-Authored-By: romanrizzi <romanalejandro@gmail.com>
Co-Authored-By: jjaffeux <j.jaffeux@gmail.com>

db/fixtures/007_web_hook_event_types.rb

@@ -37,3 +37,8 @@ WebHookEventType.seed do |b|
   b.id = WebHookEventType::QUEUED_POST
   b.name = "queued_post"
 end
+
+WebHookEventType.seed do |b|
+  b.id = WebHookEventType::REVIEWABLE
+  b.name = "reviewable"
+end