apparmor: add user namespace creation mediation

Unprivileged user namespace creation is often used as a first step
in privilege escalation attacks. Instead of disabling it at the
sysrq level, which blocks its legitimate use as for setting up a sandbox,
allow control on a per domain basis.

This allows an admin to quickly lock down a system while also still
allowing legitimate use.

Reviewed-by: Georgia Garcia <georgia.garcia@canonical.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>