MSC4140: Cancellable delayed events

[toger5]

Rendered

This could also supersede MSC2228 (by making it possible to send a redaction with the /send endpoint. This is the case as mentioned here)

Implementations:

• [ ] Synapse
  • https://github.com/element-hq/synapse/pull/17326
• [x] Element Call running in SPA mode
  • https://github.com/matrix-org/matrix-js-sdk/pull/4294
  • https://github.com/element-hq/element-call/pull/2529
• [x] Element Web running Element Call in embedded mode
  • https://github.com/matrix-org/matrix-js-sdk/pull/4294
  • https://github.com/matrix-org/matrix-widget-api/pull/90
  • https://github.com/element-hq/element-call/pull/2529
• [x] Element X running Element Call in embedded mode
  • https://github.com/ruma/ruma/pull/1845
  • https://github.com/matrix-org/matrix-rust-sdk/pull/3600
  • https://github.com/element-hq/element-call/pull/2529

[turt2live]

(please avoid force pushing after review has been given - it makes it harder to review. We squash commits upon merge, so hundreds of 'fix typos' is fine)