Adding a Mention model, test stubs

db/migrate/20160224223247_create_mentions.rb

@@ -0,0 +1,12 @@
+class CreateMentions < ActiveRecord::Migration
+  def change
+    create_table :mentions do |t|
+      t.integer :account_id
+      t.integer :status_id
+
+      t.timestamps null: false
+    end
+
+    add_index :mentions, [:account_id, :status_id], unique: true
+  end
+end