[Standards] New XEP proposal: Spaces

Hello everyone, I just submitted https://github.com/xsf/xeps/pull/1426 with a rendered version at https://nicoco.fr/inbox/spaces.html This is an attempt at a minimal "spaces XEP". I remember reading a document a long time ago which looked impressively complete but did not turn into a XEP proposal. I suspect this is because the document attempted to cover everything, and there are very different use-cases for spaces, with different permissions models, ranging from fully public to completely private through "some rooms are public but some require being part of a subteam" etc. I tried a different approach here, attempting to lay the foundation of what is the minimum to cluster several groupchats without relying on a dedicated MUC service just for that. The slidge-based gateways for discord, mattermost and matrix already implement listing the "servers", "teams" and "spaces" of these networks via adhoc commands. I would very much like to make this a bit more generic (and standardised!) in slidge "core", the gateway library (and also expose in the disco#info of each room which "space" it is a child of). About the name, I called it "server-side spaces" because I think what Gajim already does with its workspaces is great and should be standardized at some point too, that would be another XEP, maybe "client(or user)-side spaces", later. I already gathered a little feedback from the xsf@ room, and incorporated it in the present submission. Looking forward to reading your feedback. -- nicoco