mfossati, jdlrobson: Backport for [[gerrit:1295963|Make MultimediaViewer compatible with MobileFrontend legacy parser (T427542)]], [[gerrit:1295962|Carousel: Defer to MobileFrontend lightbox on mobile (T427679)]] synced to the testservers (see https://wikitech.wikimedia.org/wiki/Mwdebug). Changes can now be verified there.