web: Reload Cockpit when logging out, and go back to login page

The integration tests cope with this by explicitly waiting for the
reload to happen and reinjecting their testing code into the browser.

Fixes #512
Closes #596
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
Signed-off-by: Marius Vollmer <marius.vollmer@redhat.com>
 * Fix integration tests.