shell: Add cockpit.confirm function

Reviewed-by: Stef Walter <stefw@redhat.com>
shell: Use cockpit.confirm when deleting containers and images

Reviewed-by: Stef Walter <stefw@redhat.com>
shell: Use cockpit.confirm when erasing storage things

Also, put a warning into the format dialogs.

Fixes #73
Reviewed-by: Stef Walter <stefw@redhat.com>
shell: Include user name in confirmation dialog when deleting an account

Closes #795
Reviewed-by: Stef Walter <stefw@redhat.com>