add "@attendants" as pseudo-group for notifications

[Fusselwurm]

…because that would be sooo awesome.

• write a post in event thread
• add "@Attendants" as string somewhere
• everyone attending gets a notification about the post, as if they had been mentioned by name (or group)

[Fusselwurm]

:/

• To add an ephemeral group or pseudo-group to the autocomplete, I'd have to hack nodebb-plugin-mentions… which currently selects all visible groups ( Groups.getGroups('groups:visible:createtime', …)
• alternatively, I could try to add an actual "attendants" user to the board 😒 .