Onboarding: if a team member is added, add them to 5 recent relay groups and resend past messages for context

[missytake]

Adding new team members to old relay groups only makes sense if they can also see the past context; resending messages would be perfect for that.

The resending should happen for any user who is added to a relay group by the team, no matter if they were also just added to the team itself.

According to https://c.delta.chat/classdc__context__t.html#ad63ba1f955c677ccca143d1d12913be8, the bot can only resend messages sent by itself - this is pretty useless for conversations where there has been at least one outside reply from the team.

But IIRC, adb also lets some group bot resend the last 10 messages.

[missytake]

adb says it's not possible - mostly because even if the bot re-encrypts the messages, which are not from itself, to the new user, it can't sign them in the name of the original sender.

So an alternative would be, if (a) new member(s) is/are added to a relay group:

• the bot posts an explanation message to the group
• the bot forwards the past 5 messages which were exchanged with the outside for context to the relay group

And if new member(s) is/are added to the team:

• the bot posts a message to the team that they are added to the past (up to 3) relay groups, and if their presence is required in more relay groups, they should be added manually.
• add the new team member(s) to the recent (up to 3) relay groups

This way, noise is reduced to a minimum.

Problem: even if a user adds multiple group members at once, this creates one member-added message per contact. It would still be nice if the bot would catch this and add them all at once, only forwarding the new messages once; maybe waiting 10 seconds for more "member-added" messages before actually adding them to the relay groups would be nice.