mediawiki.api: Fix getToken() incompat with jQuery 3.0
* getToken()'s fail() handler deletes property from promiseGroup.
However sometimes this doesn't exist yet for the current api url.
Move the block that creates this object higher up.
* Deferred.then() used to have an undocumented 'this' context.
No longer exists in jQuery 3.0, but we weren't supposed to be
using returning it as-is anyway.
Return a new Deferred instead, like we do elsewhere.
Bug: T124742
Change-Id: I52c6bb779c5e79161e38a5d0c7c2cb3b458be9ba