Fix mediawiki.ui.checkbox loading in mobile

* mwui checkboxes can load on mobile, too

* mwui should have position on top to load before the content is rendered

Follow up: I6a0db7c8ce33d242120f1cba9222db4e2154696c
and I89d80927d603407678182592a1b12e2132de4141

Change-Id: I8222610aa714f7e422acaa3c3041c41f817eab7d