We shouldn't allow IP addresses as Matrix "hostnames" on the open internet

For testing and local servers, sure, but not on the open internet. This leads to a number of problems:

- Proposed methods of "ending" federation (via DNS records) isn't possible, meaning servers will talk to them until the end of time unless someone intervenes
- It's a trivial DoS vector (get an IP, set up a server, join a bazillion rooms, release the IP)
- It's a trivial spam vector
- It's also confusing from a user perspective (where mxids can be thought of as analogous to an email address)

We should only allow IP addresses (probably with a config flag for servers) that are within the local use ranges, for local-network and testing purposes, and then blacklist the rest.

No file, test, or entry point is named. Start by reviewing the Matrix specification's current hostname and federation rules, then trace how IP addresses and local-use exceptions are defined; done means the specification clearly distinguishes permitted local/testing addresses from public Internet addresses and documents the proposed configuration behavior.

Written by the indexing model from the issue text.