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>

spec/fabricators/reviewable_fabricator.rb

@@ -0,0 +1,58 @@
+Fabricator(:reviewable) do
+  reviewable_by_moderator true
+  type 'ReviewableUser'
+  created_by { Fabricate(:user) }
+  target_id { Fabricate(:user).id }
+  target_type "User"
+  target_created_by { Fabricate(:user) }
+  category
+  score 1.23
+  payload {
+    { list: [1, 2, 3], name: 'bandersnatch' }
+  }
+end
+
+Fabricator(:reviewable_queued_post_topic, class_name: :reviewable_queued_post) do
+  reviewable_by_moderator true
+  type 'ReviewableQueuedPost'
+  created_by { Fabricate(:user) }
+  category
+  payload {
+    {
+      raw: "hello world post contents.",
+      title: "queued post title",
+      tags: ['cool', 'neat'],
+      extra: "some extra data",
+      archetype: 'regular'
+    }
+  }
+end
+
+Fabricator(:reviewable_queued_post) do
+  reviewable_by_moderator true
+  type 'ReviewableQueuedPost'
+  created_by { Fabricate(:user) }
+  topic
+  payload {
+    {
+      raw: "hello world post contents.",
+      reply_to_post_number: 1,
+      via_email: true,
+      raw_email: 'store_me',
+      auto_track: true,
+      custom_fields: { hello: 'world' },
+      cooking_options: { cat: 'hat' },
+      cook_method: Post.cook_methods[:raw_html],
+      image_sizes: { "http://foo.bar/image.png" => { "width" => 0, "height" => 222 } }
+    }
+  }
+end
+
+Fabricator(:reviewable_flagged_post) do
+  reviewable_by_moderator true
+  type 'ReviewableFlaggedPost'
+  created_by { Fabricate(:user) }
+  topic
+  target_type 'Post'
+  target { Fabricate(:post) }
+end