ws: Add new cockpit_webserver_new_table() function

Returns a GHashTable with ASCII case insensitive comparison of keys.
For now, no duplicate headers supported: we don't need them.
ws: Separate HTTP resource handlers into own file

This makes them testable. Add tests for the /login handler
ws: Add function to simply return static content to client
ws: Handle static HTTP resource requests from cockpit-ws

This is a simpler architecture.

We also try not to use long lived DBus proxies for handling
unauthenticated requests.