Allow loading of Angular routes inside tabs

Since much of the front-end is moving toward Angular, it makes sense to allow Angular forms to be embedded in non-Angular pages.

Here's a use case: Ginkgo Street Labs is using Angular to build a front-end for CiviVolunteer projects which are not associated with events. The pages for managing events are all PHP-driven. As we add more volunteer project configurations, it makes sense for us to be able to replace the form in the volunteer tab of an event with the Angular form in the new user interface. Otherwise we'll end up maintaining parallel forms for the same purpose: one written in PHP, the other written in JS.