web: Use bar graphs in container image overview rows

These represent the size of the image on disk.

Partially fixes #312

Signed-off-by: Marius Vollmer <mvollmer@redhat.com>
web: Added cockpit_get_dbus_client function.

To allow multi-machine aware pages to do the right thing.

Reviewed-by: Stef Walter <stefw@redhat.com>
web: Memory usage bar graphs in container overview rows

Partially fixes #312

Reviewed-by: Stef Walter <stefw@redhat.com>
web: Add CPU column to container overview.

Closes #316
Fixes #312

Reviewed-by: Stef Walter <stefw@redhat.com>