mediawiki.user: Return a promise from getRights/getGroups

Makes it easier to work with when trying to execute multiple
asychronous tasks in parralel with $.when, otherwise one would
have to wrap it in a custom Deferred again to be able to
aggregate it as a promise.

Change-Id: I69a132a2e6f2ddb236bc1a0dbd8f7ab311f7f873
Merge "mediawiki.user: Return a promise from getRights/getGroups"