Minimize the chat state/complexity

[quartzjer]

During testing the current chat channel proved far too complex to be a minimal utility, this is an attempt to simplify it even further.

• a stateless join channel for inviting and the joining process
• a profile message type that is attached to joins
• join messages are only sent by the leader
• all messages happen within a chat channel
• all participants must always be in sync

[quartzjer]

I'm very happy with how this turned out, the implementation is significantly improved and was able to (re) do it in a few hours.

[quartzjer]

Few more cleanups in now, going to merge this shortly...

[dvanduzer]

Not introduced in this commit, but is the weird escaping intentional? https://github.com/telehash/telehash.org/blame/master/v3/channels/chat.md#L102