consider the user's busy status when assigning issues

it might be good to skip (or rank down somehow) users that are marked as "busy", so that users have a way to schedule themselves AFK, when they go on vacation for example.

obviously, it could backfire if people forget to unset the busy status when they return, but it seems still worthwhile to look into.

i explicitly do not mention the idea of writing a bot which rewrites the ruby code based on nextcloud calendar status. that way lies madness. (although having a sync between the "busy" status and the nextcloud calendar would surely be nice, but that's still a separate issue.)