Add option to disable two factor auth in admin accounts panel. (#2584)

* Add option to disable two factor auth in admin accounts panel.
Closes #2578

* Add @mjankowski's suggestions.
* Moves destroy actions behind User#disable_two_factor!
* Adds spec coverage for Admin:TwoFactorAuthenticationsController and User#disable_two_factor!

spec/controllers/admin/two_factor_authentications_controller_spec.rb

@@ -0,0 +1,17 @@
+require 'rails_helper'
+
+describe Admin::TwoFactorAuthenticationsController do
+  render_views
+
+  let(:user) { Fabricate(:user) }
+  before do
+    sign_in Fabricate(:user, admin: true), scope: :user
+  end
+
+  describe 'DELETE #destroy' do
+    it 'redirects to admin accounts page' do
+      delete :destroy, params: { user_id: user.id }
+      expect(response).to redirect_to(admin_accounts_path)
+    end
+  end
+end