Fix #91 and update frontmatter structure of events files

[niklasmtj]

This PR fixes #91 and introduces the discussed format of:

---
title:
date:
location:
- url: 
+ attend:
+ recordings:
---

The events section will only show the attend or recordings button when an URL is found in the respective .mdx event file.

This will result in the following event section:

@scottrigby @christianh814 please check if the recordings are correct. I used the ones I found in the file but a couple of these were from Meetup which did not hold any recordings links.