Add a new client side form.js library which is a more or less direct
reimplementation of the Lua side cbi.lua in JavaScript.
Due to its client side nature, it supports a number of features which
would be hard to realize on the server side, such as drag&drop sorting,
modal sub-map dialogs, reload-free editing etc.
Signed-off-by: Jo-Philipp Wich <jo@mein.io>
