luci2: major changes to RPC implementation

	- rework LuCI2.rpc namespace to support batching arbritary requests
	- use new LuCI.rpc.declare() generator function to declare RPC proxy functions
	- thoroughly use jQuery promises for chaining RPC responses, drop callback model
	- drop internal rcall() and retcb() helpers in favor to the new RPC implementation
	- adjust existing views for new RPC model
	- implement session alive check to automatically prompt for a login if the server session is expired