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/migrate/20190110212005_create_reviewable_histories.rb

@@ -0,0 +1,14 @@
+class CreateReviewableHistories < ActiveRecord::Migration[5.2]
+  def change
+    create_table :reviewable_histories do |t|
+      t.integer :reviewable_id, null: false
+      t.integer :reviewable_history_type, null: false
+      t.integer :status, null: false
+      t.integer :created_by_id, null: false
+      t.json    :edited, null: true
+      t.timestamps
+    end
+
+    add_index :reviewable_histories, :reviewable_id
+  end
+end