Spec /relations and aggregations

[turt2live]

• MSC3676 (MD) - Reply fallbacks relaxation
• MSC3666 (MD) - Bundling/aggregations for search
• MSC2674 (MD) - Relationships
• MSC2675 (MD) - Server-side aggregation

Note: Originally this branch was intended to also carry MSC3440 (MD; Threads), however it was determined that in order to include threads we'll need a bit more of the feature to land first. There shouldn't be any lingering references to threads (at least in a "threads are specced" capacity) - please point them out if they are still there.

Reviewer: The diff is a bit messy and the commit history isn't much better, sorry. To summarize what has happened: the rich replies stuff got broken out to a dedicated module (in preparation for Threads getting a dedicated module itself) and the relationships stuff got airlifted to a higher part of the spec. Then, edits to all the sections to make the text clear/match the MSCs.

Preview: https://pr1062--matrix-spec-previews.netlify.app

[turt2live]

@clokep github isn't letting me re-request review from you for some reason, but if you get a chance then a second look would be appreciated (non-blocking, though).

[clokep]

Seems reasonable to me, minus the couple typos! 👍

[turt2live]

thanks for the review, and apologies again for the dumpster fire that is a commit history on this. Hopefully will be improved for future MSC stacks.

I think your suggestions generally make sense - will take a closer look hopefully tomorrow and unpick pagination in the process.

[turt2live]

@richvdh this should be ready for re-review, with the commits since your last review being reviewable individually (largely).

[richvdh]

thanks for the updates on this!

[richvdh]

let's ship this at last