mediawiki.api: Fix getToken() incompat with jQuery 3.0

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