Adding unified streamable notifications

db/migrate/20161119211120_create_notifications.rb

@@ -0,0 +1,13 @@
+class CreateNotifications < ActiveRecord::Migration[5.0]
+  def change
+    create_table :notifications do |t|
+      t.integer :account_id
+      t.integer :activity_id
+      t.string :activity_type
+
+      t.timestamps
+    end
+
+    add_index :notifications, :account_id
+  end
+end