Merge and remove Warning on Helmholtz AAI maintenance/outage

2021-10-25:

• Merge for hifis.net (!316 (merged))
• Merge for aai.helmholtz.de (https://gitlab.hzdr.de/helmholtz-aai/aai.helmholtz.de/-/merge_requests/23)
• Add in events.hifis.net

2021-10-26 after 10:00

• Remove for hifis.net
• Remove for aai.helmholtz.de
• Remove in events.hifis.net