BUG11278: It is not possible to create subnets of internal networks in firewallgroups

BUG11278: It is not possible to create subnets of internal networks in firewallgroups

Fixes: #11278
When creating networks which are part of an internal network, there was an errormessage displayed and the creation was prohibited.
Now it is possible to create such subnets. This is used at own risk! Users have to take care of the firewallrule sequence.
It is possible to create situations that are not wanted.

Signed-off-by: Alexander Marx <alexander.marx@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>