Calendar: Sync with user devices

[pantierra]

Idea: allow people to subscribe to the calendar by adding a link on the website's calendar.

This is a follow up to #45: Include CalDAV link to the calendar

[pantierra]

Started looking into this briefly:

• An ics file doesn't seem to be suitable as it imports only the state of a given time, and is not able to sync with updates that are constantly happening in the calendar.
• CalDAV could work, but seems to need authentication and access to the following endpoint:
  • https://files.x-hain.de/remote.php/dav
  • https://files.x-hain.de/remote.php/dav/principals/users/USERNAME/ (for MacOS)

We need to find a way to subscribe to the calendar without authentication and then add the link.

[pantierra]

Nevermind. Found a working public link and will add it in a PR.