Replace and improve calendar

[pantierra]

Currently, the calendar is just an iframe pulling from Nextcloud. We would prefer a more native handling of the calendar. @mrhide-de started a great prototype of an xhain calendar. Now it is time to bring this to our website.

First step to a minimal viable product:

• Download and parser for iCal file
• Rendering as HTML within the website
• Basic list of events
• Setup infrastructure to run build process every night
• Introduce month view

Ideas for further improvements (ideally one PR for each)

• Make month info stay on top
• Clickable links in modal
• Include CalDAV link to the calendar
• Introduce recurring rule information
• Translation of calendar
• Improve accessibility

[pantierra]

For the record, @jdwillemse added the following to the calendar with #63 (merged with #64) 👏

• Translation of the calendar (not the events of course)
• Make month info stay on top

Created follow-up issues:

• Calendar: Sync with user devices #65
• Calendar: Clickable links in modal #66