Support threads with m.thread relation (MSC3440)

[jaller94]

Threads are being added to the Matrix protocol soon. Because Slack already supports threads in its protocol and interface, it would be good to bridge that correctly. Currently, Slack threads are bridged as replies.

Related issues:

• Meta issue: Threads P0: Web element-hq/element-meta#3
• MSC3440: MSC3440: Threading via m.thread relation matrix-spec-proposals#3440

Definition of Done

• Slack threads are bridged to Matrix as m.thread relations – removing the reply relation we currently set.
• Matrix threads are bridged to Slack as threads.

[abmantis]

What would be the behavior in clients that do not implement threads yet? Will they still fallback to replies?

[jaller94]

The fallback will like be that the messages are shown without context in the main thread – not as replies.
This is unlikely to affect the plan of implementing this in a bridge once it's in the stable Matrix Spec.

[abmantis]

Will this be only implemented when this gets into the stable spec? :(

[Half-Shot]

We're planning to implement this sooner.