Enable useful event IDs/slugs

See https://gitlab.hzdr.de/hifis/software.hifis.net/merge_requests/159#note_25630 & https://gitlab.hzdr.de/hifis/software.hifis.net/merge_requests/21#note_19914

It is IMHO crucial for effectively organising multiple future events to have each event file to have a useful, findable ID/slug/prefix. Carpentries use ISO-8061 of the event's 1st day on the repo/folder level.

Since currently, our Jekyll refuses to build files with future date slugs in their filename, we have to use past or present date slugs which are not meaningful for the event orga.

Therefore, this change is a suggestion & attempt to fix that problem. Not tested locally.

If agreed

• revert b85204eb

After merge:

• @katrinleinweber: wait for https://gitlab.hzdr.de/hifis/software.hifis.net/merge_requests/164, https://gitlab.hzdr.de/hifis/software.hifis.net/merge_requests/163 & https://gitlab.hzdr.de/hifis/software.hifis.net/merge_requests/159, then update filenames in https://gitlab.hzdr.de/hifis/software.hifis.net/merge_requests?scope=all&utf8=%E2%9C%93&state=opened&search=announce