webtrees theme redo top menu...

... to fit on a single line on a 1024 wide screen when all standard menu items are enabled. Uses flexbox where
possible and falls back to table display for older browsers.

This is probably what should have been submitted for webtrees theme on commit https://github.com/fisharebest/webtrees/commit/df15c0ba4477c857fdddfe30480188bdde98004d#diff-8dda8034bd86e7c10a160c85f4fb6932