[rebuild] [2.3] Enable usernav menus for mobile devices

*(Updated PR to target `master`)*

Should address #11600, #11704 and most of #11706 *(menus aren't easily closed on mobile per `:hover`)*

Now dropdown menus from usernav (right-hand side) are accessible from mobile devices. Also, mirrored topnav styles to usernav (cursor: default), but maintain cursor: pointer for top menus (containing children) with a defined action...e.g. for actual links.