Return jqXHR from mw.Api calls

Exposes the jQuery XHR object to mw.Api clients. This is needed
for logging cache headers in MultimediaViewer, and surely has
other uses as well.

Change-Id: I49977b5f9d155cc7332377c906a31f5689bec8b4
Merge "Return jqXHR from mw.Api calls"