luci-app-firewall: disable port fields when protocol is not TCP or UDP

It's currently possible to generate nonsensical firewall rules by inputting
combinations which include:

i) protocols other than UDP/TCP
ii) source and destination ports.

There is some discussion of the issue on the forum here and the issue is
here; #1850.

This patch makes fields like src_port and dest_port depend on protocol being
tcp, udp or "tcp udp" in the input, forwarding and source NAT forms.

Signed-off-by: Tom Hodder <tom@limepepper.co.uk>
[reword commit message, squash commits]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>