mediawiki.page.gallery.slideshow: Avoid manual Deferred wrapping

mediawiki.page.gallery.slideshow: Avoid manual Deferred wrapping

* Use then(), which automatically forwards errors, instead of manually
  forwarding fail() to d.reject(). This also ensures that any
  errors in the done() or fail() handler themselves are caught and reject
  the returned Deferred – instead of an causing the Deferred to indefinely
  be in the "pending" state.

* Use then(), which makes the code consistently asynchronous,
  instead of sometimes sync and sometimes async.

* Use then() for forward-compat with native ES6 Promise.

Change-Id: I58fb6ec23fb4056fe2bbb146ff4225dea9d9f379