Aggregations: Use backend support

[jryans]

This is a placeholder issue for things like:

• Applying bundled relations from sync
• Paginating relations

At the moment, Riot Web does all this client-side without assistance from the server, but this likely means we'll miss relations when there's a gappy sync, etc.

[jryans]

At the moment, it feels hard to be sure how to interpret the bundled relations correctly.

In the case of reactions for example:

• If you scroll back and get the target event with bundled reactions, you don't know which reaction events are included, so you don't know whether to add / remove for later events in the timeline (MSC comment)
• Bundled data doesn't currently include reaction senders which we show in the hover tooltip (MSC comment)

[jryans]

Marking as blocked until these spec concerns have a clear resolution.

[bwindels]

Related: #10330