RFC2317-style reverse zone from prefix

[krejcar25]

Currently, DNS UI can create, display and work with a reverse zone in the RFC2317 format. However it can not create a zone 4/30.3.2.1.in-addr.arpa from the prefix 1.2.3.4/30, nor can it display the prefix and subnet in the respective columns in the zones list. I'd like to see this feature.

I haven't looked at the code, I'm useless with PHP, but if it would be mostly copy-pasting code, I might be able to cobble together a PR.

Also, how should be the two columns rendered? Only thing that comes to mind is something like this:

Zone name	IPv4 prefix	Subnet	...
4/30.3.2.1.in-addr.arpa	1.2.3.4	1.2.3.4/30	...

but that makes the two columns almost identical. Any ideas on how to use both of them better?

Lastly, I feel like this might even help implement #24, but I imagine that would require more PHP coding than this feature.