Drop RSVP dependency

[jelhan]

The rsvp dependency should not be needed anymore. It was primarily a polyfill for native Promise. All modern browsers support native Promise these days. We can simplify our code base and prepare for planned deprecation by removing the dependency.

@NullVoxPopuli started the work in #1875 but doesn't have the time to finish it.

RSVP is not used at many places. GitHub code search lists 11 places.

Main priority should be on code shipped to end user. This affects 3 places:

1. transitionEnd utility function (addon/utils/transition-end.js)
2. <BsForm> component (addon/components/bs-form.js)
3. Blueprints (blueprints/ember-bootstrap/index.js)
4. Having rsvp as a dependency rather than a dev dependency.

The other 7 places affects only code, which is not shipped to consumers:

• docs app (2 files)
• tests (4 files)
• node tests (1 file)

In most cases the transition should be fairly simple. RSVP.Promise, RSVP.reject, and RSVP.resolve can be replaced with their native equivalents. Only RSVP.defer does not have a native equivalent. But it could be replaced by an utility function, which I sketched here: #1875 (comment)