Add `loaded` event to modals that use remote

Fixes issue #5169. Uses the callback in jQuery's `.load()` to trigger a `loaded` event on the modal element.

I am resubmitting this. It was previously pull request #6175. Since that pull request was closed, a number of people have voiced support for the addition of a `loaded` event:

* [@jordanisip](https://github.com/twitter/bootstrap/pull/6119#issuecomment-11014365)
* [@rmm5t](https://github.com/twitter/bootstrap/pull/6175#issuecomment-11979098)
* [@cvn](https://github.com/twitter/bootstrap/pull/6175#issuecomment-12352770)
* [@Nerian](https://github.com/twitter/bootstrap/issues/5169#issuecomment-11708043)
* [@pixelcode](https://github.com/twitter/bootstrap/issues/5169#issuecomment-12963204)
* [@stylenclass](https://github.com/twitter/bootstrap/issues/5169#issuecomment-13074384)

Also, please see [my justification](https://github.com/twitter/bootstrap/issues/5169#issuecomment-11639499) for adding this single event.