Support threads in Matrix (MSC3440)

[jaller94]

Fixes #634

Note: Slack doesn't seem to have a separate UI for replies. Everything is a thread.

Definition of Done

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

[jaller94]

Known issues (I'd like to file these as issues (if not already filed) and solve in different PRs):

• Slack's feature "Also send to [main timeline]" isn't supported in Matrix. Messages will only show up in the thread.
• Sending images and other files does not work in threads.
  • To be fair, they currently also don't work with Matrix replies. It's not a regression.
  • Slack -> Matrix: The image arrives in the main timeline, not in the thread.
  • Matrix -> Slack: The message appears in the thread, but is empty.

[Half-Shot]

Slack's feature "Also send to [main timeline]" isn't supported in Matrix. Messages will only show up in the thread.

Can we do a dumb thing where we just send the event twice to Matrix? Would this be too spammy? Alternatively should we comment on the proposal to raise the gap in the spec?

Sending images and other files does not work in threads.

I'd be reasonably okay with leaving this as an open issue to fix, if it's not an easy fix.

[Half-Shot]

Code looks super clean and appears to work, nice one!

[Half-Shot]

So I'm not sure if you still need a fallback or not, looking at what Element does:

[jaller94]

Element does not provide this fallback.
I guess if a client or bridge (e.g. IRC) wants to display the relation, it up to that cleint's or bridge's implementation to provide an adequate representation.