Edit the metadata of multiple events

[Melanie-Koller]

If i like to change the date of multiple events with the same date, there is written "invalid date". And it is only possible to change the date with the calendar. I can't write the date.

[Arnei]

#606 should make it so that "Invalid date" is not wrongly shown anymore.

On the most current version of the admin ui I am able to change the date both by calendar and by typing, so maybe that part of this issue was already fixed incidentally.