Simplified Sliding Sync

[dbkr]

Replaces the sliding sync option in labs with Simplified Sliding Sync so it will support the impl in Synapse directly rather than via the sliding sync proxy.

Anyone who had the old labs flag turned on will find that their client breaks very abruptly when updated: it will simply throw an error when it tries to start syncing. This is done deliberately to fail fast since otherwise it could cause subtle crypto failures with both a proxy and the client syncing directly. matrix.org has not run a sliding sync proxy for some time but other homeservers may still be running one.

This is still a way from perfect but getting it into labs is a place where we can start getting it ready for primetime.

Requires matrix-org/matrix-js-sdk#4400
Ported from element-hq/matrix-react-sdk#36

Checklist

• Tests written for new code (and old code if feasible).
• New or updated public/exported symbols have accurate TSDoc documentation.
• Linter and other CI checks pass.
• I have licensed the changes to Element by completing the Contributor License Agreement (CLA)