Allow custom base-pages with less `crmApp` boilerplate

The AngularLoader is a helper class for setting up a custom Angular base-page.

https://docs.civicrm.org/dev/en/master/framework/angular/loader/#other-base-pages

The pages "civicrm/a", "civicrm/case/a", and "civicrm/vol" all have similar requirements for how they load (e.g. they only load Angular, and it's the main content of the page). But there's little explanation/documentation/tooling for handling Angular bootstrap (`<div ng-app>` or `angular.bootstrap()`) in this case.

We should add and document a helper function to `AngularLoader()` which addresses this common case.

NOTE: This would not be universally applicable. For example, CiviCase injects Angular into a subsection of an existing screen ("View Contact"), and that requires a somewhat different approach. So it should be optional.