Add calendar/event list

[sbruder]

It is implemented by making it possible to assign event metadata to an announcement like this:

---
title: Test announcement
date: 2022-07-18
description: "This will be shown in the iCal event"
extra:
  event_start: 2022-07-19T10:00:00Z
  event_end: 2022-07-19T11:00:00Z
---
Description of the announcement/event only shown on the website

If it is not a good way to have overlap with the announcement (which has the advantage that they are also visible on the main page), we could also separate it.

The calendar overview page is very simple, because implementing a calendar display from scratch is not very easy.

Since there currently are no calendar events, I added some fake ones to this test deployment: https://dapper-swan-e898e3.netlify.app/

[github-actions[bot]]

🚀 Deployed on https://62dbca9c95186a7b69e88ac5--nixos-summer.netlify.app

[github-actions[bot]]

🚀 Deployed on https://62dfa0fa2149a3510d95620a--nixos-summer.netlify.app