Refactor smallscreen nav to off-canvas nav (basics).

– necessary layout changes
– move toggle nav button
– incorporate user menu back into main nav
– basic styles required for off-canvas nav
– adapt typography, remove some redundant styles
– adapt toggle JS
Some stylesheet maintenance.
Bring back the button links, some minor CSS fixes.
Bring the user menu back to the top bar on desktop.
Merge branch 'off-canvas-nav' into 2.0