luci-theme-openwrt: rework and polish OpenWrt theme

 - polish styling
 - move main menu to the left for large resolutions and collapse into
   dropdown header bar for small mobile resolutions
 - make tables responsive by utilizing extra attributes to turn rows
   into wrappable flexboxes for low resolutions
 - mostly get rid of button icon references and use a uniform color
   scheme for action-, positive, negative and neutral buttons

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