MobileFrontend has its own, completely separate, implementation of the notifications popup. This means the mobile site didn't break when we OOUIified the notifications popup on desktop, but it also doesn't get anything new (and so e.g. cross-wiki notifications won't work there). We should instead reuse the OOUI widgets that the desktop site uses.
Related Gerrit Patches:
|mediawiki/extensions/Echo : master||[wip] Adjust and clean up styles and behavior for mobile|
|mediawiki/extensions/MobileFrontend : master||OOUIfying Echo in MobileFrontend|
|mediawiki/extensions/Echo : master||Add MobileNotificationsWrapper for Mobile display|