base: Move spawn.js and channel.js into base package

So they can be used from multiple packages. We also document them
in the guide and clean up the API a bit.

One minor change is that channels now automatically detect which
host they should connect to based on the page parameters.
base: Move common styling and images into base package

This renames the shell specific CSS into shell.css and splits out
the common CSS into cockpit.css
terminal: Put the terminal into its own package and task

It might not necessarily stay in its own package, but this is
a good testbed for our first package. It will however stay in
its own task.

Closes #1314