Add support for uwsd / WebSocket operation

[jow-]

This PR introduces a number of changes into the tree:

• Package the uwsd HTTP/WebSocket server
• Add client side support for requesting JavaScript class files and calling ubus RPC procedures via a WebSocket connection
• Add a new luci-uwsd collection shipping appropriate default configuration and uwsd server backend for LuCI

This is a work in progress and currently experimental.

[Neustradamus]

@jow-: Have you progressed on your PR?

[systemcrash]

Hi @jow- - shall we rebase and merge this so it sees some testing?

[jow-]

Yes, please do and give it a try.