luci-theme-bootstrap: various tweaks and cleanups

 - introduce new variable --text-color-highest which results in #000/#fff
   for bright and dark modes respectively
 - drop unused navigation style rules and selectors
 - use sticky positioning for header bar and drop top margin hacks
 - use flex box layout for header bar contents
 - use uniform line-height for button elements instead of pixel paddings
 - fix too bright .cbi-value bottom border in mobile dark mode
 - avoid rendering duplicate .cbi-value bottom borders for nested sections
 - simplify header markup, get rid of unused container elements
 - use non-wrapping flex box layout for page actions, prioritize primary
   action button in automatic width calculation
 - fix border radius of ifacebox headers
 - use brightest color for log output textareas
 - fix dark mode colors for mode menu
 - use flex layout for footer

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