luci-base: firewall.js: add getZoneColorStyle() helper

The getZoneColorStyle() function will produce CSS style properties that
describe the color value of the zone. The color declaration is divided
into a CSS variable called `--zone-color-rgb` which holds the RGB value
of the color and a `background-color` property assigning these values
as background property.

This allows themes to override the color with derived values, e.g. by
applying an alpha channel.

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