luci-base: add ubus-http gateway

luci-base: add ubus-http gateway

Add an admin/ubus route mimicking the native uhttpd-mod-ubus protocol.

The main difference to the native protocol is that this gateway requires
no additional per-object/procedure ACL setup on the router side and that
it is located under the same prefix as LuCI itself, allowing the reuse
of the session login cookie.

This route is meant to be a transitional mechanism until client side
RPC calls are eventually migrated to uhttpd-mod-ubus completely.

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