disabled, is-required and is-validating classes rendered on .form-group element do not exist in Bootstrap

[jelhan]

I noticed that .form-group element rendered by <Form::element> has some classes, which aren't existing in Bootstrap: https://github.com/kaliber5/ember-bootstrap/blob/ce9f350afd8001aa005fd5b5ace4850ee3480e9b/addon/components/bs-form/element.hbs#L3

.disabled and .is-required classes were both added in #71 a long time ago. .is-validating was added around the same time by #70.

.is-required and .is-validating classes do not seem to exist in Bootstrap 3 and 4 at all. .disabled class exists and is used in the docs a lot. But I wasn't able to find any usage on the .form-group element neither in Bootstrap 3 nor 4 docs.

I think we should remove all three. But as that would be a breaking change we can not do it before next major release.

Theoretically we can deprecate them by adding an optional feature, which conditionally removes them. We could print a deprecation warning if it's not explicitly enabled. But I'm not sure if it makes sense to spent the time on it.