Fix rt crashers

 * 0fe59757f0c8e8d4227d072360b34d232e2c1c41 introduced a bug where
   data-mw wasn't being migrated between docs in the domFragment
   unpacker.

 * Here we generalize that process a bit since we're doing it for
   data-parsoid already.

 * Ex. _rt/hewiki/רחל_בוימוול?oldid=15504362

Change-Id: Ic7827bfb83b9ea01406628e1c20f216bfb85ef64