Relay support

[alphatownsman]

• Admin UI to subscribe/unsubscribe from relay server (use inbox mode)
• Fan out public posts to active relay servers

[alphatownsman]

@andrewgodwin any suggestion to get this merged?

[andrewgodwin]

Well the problem is that we've now reached my cutoff time for keeping working on the project if nobody turns up to help full time, so there is a chance this may never get merged. It'll definitely have to wait at least until there's another maintainer who can help review it, sadly :(

[AstraLuma]

Given the vibe of relay discussion, this seems like a surprisingly small change? Does this cover inbound, outbound, and handshaking?

Are there any documents describing the relay protocol?

[alphatownsman]

@AstraLuma yes it should cover all relay functions.

There are some fundamental protocol issues we discussed previously preventing it from working with certain relays and peer servers, like the infamous as:Public issue, but i think we should track that in different PR.

I have been running this no problem on my own server for two weeks, the only issues are very occasionally some less known server implementations may send some weird message json breaking Post parser but thats not relay issues and should be fixed separately.

There are some doc in #641

[AstraLuma]

Ok, thanks. I'm going to take some time to go over this carefully because the state machines smell of fiddly and precise.