luci-theme-bootstrap: various style tweaks

 - Make control-group containers flex and ensure proper spacing between
   elements, fixes e.g. unwanted break for reveal/hide password button
   on narrow mobile views

 - Decrease top margin of modal popups

 - Fix displaying cell titles on mobile wrapped table rows

 - Tune mobile flex table styles

Signed-off-by: Jo-Philipp Wich <jo@mein.io>