Convert admin section controllers to ES6 modules

Basically whatever I could get done during the Toronto Hack Night (reference: https://meta.discourse.org/t/javascript-es6-modules-conversion/17880). Seems to work OK in quick smoke testing.

In addition to the conversions themselves – which were relatively straightforward, except where there were multiple controller definitions in one file – there were minor changes to `Discourse.Resolver` and to one of the QUnit helpers in order to get these working.