ui: firewall: rule: maxlength for source and dest

ui: firewall: rule: maxlength for source and dest

Limiting the length of the source and dest paramters helps to avoid
problems with iptables-restore which would not apply a rule if a
parameter is larger than the parameter buffer (1024)[0]. As the API is
already limiting this, we should also reflect that in the GUI and give
people a hint that IP sets are most likely the better approach.

[0] http://git.netfilter.org/iptables/tree/iptables/xshared.c?h=v1.8.7#n469

Signed-off-by: Aaron Lauterer <a.lauterer@proxmox.com>