luci-base: introduce common JavaScript api

Introduce a new script file luci.js which is included by default and
intended to be the common location of functions currently scattered
in cbi.js and xhr.js.

The luci.js file provides a LuCI() class which - among other things -
implements helpers to construct URL paths and making HTTP requests.

A singleton instance of the class is instantiated as window.L upon
load and preset with the necessary environment information.

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