Visualise event registration status
@mdolling-gfz's idea, see https://gitlab.hzdr.de/hifis/software.hifis.net/issues/70#note_22697
Interested learners currently need to visit the individual event's page to learn whether they can still sign up or not. We could implement some kind of flag to show the event status, like "open", "wait list", "over", etc. so that they immediately where they can still register.
Alternatively, we could hide any non-open events, grey their entry out or move them to an archive area, to avoid adding a visual hint, but still making it clear which events they no longer need to hope for.