make csrf error page translateable, fix HTML errors in csrf.inc

Groups are only used as firewall rules interfaces, so move them
into the "pf" "plugin" and fix the menu accordingly.

(plugins, interfaces) add ifgroup type, filter in interfaces_groups_edit.php. for https://github.com/opnsense/core/issues/1455

(gwlb.inc) more cleanups, $ifname == $gateway_item['friendlyiface'] looks kind of impossible here, $ifname is unique and not in $gateway_item yet...

It's a bit funny that the help text indicates we can use tag/tagged with
NAT rules as well, because this was never implemented although it really
works.

(firewall) empty means "in"... confusing ;) https://github.com/opnsense/core/blob/17.1.2/src/etc/inc/filter.inc#L2314-L2315

(firewall) add port/address parsing to FilterRule(), should be backwards compatible with current config settings

(filter) filter_generate_port, why all this trickery?? it's either a valid port range or it's not. not need to overreact.

It's better to replace this now, the new style code won't include this "magic".