base: Move jquery bootstrap and term.js into new base package

And make the shell use this package.

The module is called 'base' but we go by the alias 'latest' which
contains the latest unstable Cockpit API. There will be counterpart
modules in the future.
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.

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.