Allowlist for private IPs

[HelgeKrueger]

For testing/development purposes, one needs to be able to allowlist private IP addresses. Such an allowlist is better practice than the current simple yes/no choice.

For implementers:
dahlia/hollo#53 (comment)

and the entire discussion therein for background.

[ThisIsMissEm]

The only complexity I foresee here is that we're not using a package for IP address handling, but actually some inlined code (this concerns me a bit)

As such we don't have easy functionality to test ranges against IP addresses