[Enhancement] Add option to disable automatic grouping of private IPs

[schlagmichdoch]

I'm trying to use PairDrop on an internal network that spans multiple 10.0.0.0/16 networks. X-Forwarded-For is correctly reporting the 10.x.y.z IP for clients, but ipIsPrivate assumes all private IPs are on the same network.

I don't know enough to suggest a good solution, but a good solution does seem difficult.

Just wanted to add my (hopefully not unique) use case for potential consideration.

Originally posted by @Skinner927 in #310 (comment)

We could solve this by adding an ENV VAR:

• DETECT_PRIVATE_IPS:
  Specify whether all devices with private IPs should see each other mutually
  • DETECT_PRIVATE_IPS=false: Switch this off
  • DETECT_PRIVATE_IPS=true: Switch this on (default)

@Skinner927 Would this solve your issue? Devices would then only be able to find each other by pairing or entering the same public room. If your PairDrop instance is not reachable from the internet you could also take a look at the proposed ENV VAR to simply switch off grouping via IP: #301 (comment)

[Skinner927]

Yes, that would appear to solve it quite nicely.