Refine thread participants/subscribers

[tiltec]

We currently treat thread participants and subscribers the same, whereas in reality they can differ:

• the original author is automatically a subscriber, but not a participant (also see Number of replies in thread is indicated wrongly #1968)
• after somebody left the group, they are still a participant but not a subscriber

We could do those changes:

• API: replace thread participants with users - those are people who actually wrote replies and keep them in order of first participation. This list could be generated on demand (i.e. not kept in database)
• Database: remove users from ThreadParticipants when they should not get notifications

[github-actions]

This issue is marked as stale because it has not had any activity for 90 days.

It doesn't mean it's not important, so please remove the stale label if you like it, or add a comment saying what it means to you :)

However, if you just leave it like this, I'll close it in 7 days to help keep your issues tidy!

Thanks!