Use MobileFrontend

The code is here: https://gerrit.wikimedia.org/r/#/admin/projects/mediawiki/extensions/TwnMainPage

Adding mobile target to ext.translate.mainpage.styles and jquery.uls.grid resolved the problem (except that it’s not entirely scalable, i.e. the header is at least 600px wide, but I think that’s acceptable); however, I don’t know if there is some reason to not load the ULS grid style on mobile.

I created a Phabricator task for it so that ULS developers are also notified, and it can be linked in Git commit messages.