Remove angular promises from Courier

[stacey-gammon]

In order to extricate angular from courier, a good place to start is to get rid of the Promise classes passed everywhere (which we decided was our plan in #13855).

This isn't super straightforward though because we make use of Promise.defer which is deprecated in native javascript Promise class. I suspect using observables will help because the defer object is passed around and used to reject the promise in certain situations (like an error is encountered, or the page is navigated away from).

First step is to figure out exactly how defer is being used and write a couple tests to make sure our refactoring isn't affected by it. It's used in error handling (cancel any existing requests), and I think with auto refresh on.

cc @timroes @chrisdavies

[timroes]

cc @lukasolson I assume that will anyway happen via the courier rewrite, so just wanted to highlight this issue again, so we don't forget to close it later :-)

[lukasolson]

This has been resolved.