Move out datengartens into their own section

- /datengarten/index.html returns a blog-like list
- /datengarten/index.ics returns a VCALENDAR
- /datengarten/index.xml returns a schedule.xml-style view

No more pretty rendering includes in the content pages,
some more cleanup in the front matter

content/datengarten/72.md

@@ -6,13 +6,14 @@ no: 72
 subtitle: "Lightning Talks"
 speaker: starbug, N-te, Marko, DC4LW, Ligi, melle
 date: 2016-12-13T00:00:00+02:00
-event_date: 2016-12-13T20:00:00+02:00
+event:
+  start: 2016-12-13T20:00:00+02:00
+  end:   2016-12-13T22:00:00+02:00
 location: CCCB
-language: Deutsch
+language: de
 streaming: true
 recording: https://media.ccc.de/v/dg-72
 ---
-{{< datengarten-infobox >}}
 
 Lightning talks are 10 minute talks about *anything*. Well, anything that won't make other people want to throw you out. But still, it can be about anything that you really have to tell everybody else. An idea you have, a problem you cannot solve, your favourite tool or just about anything you think everybody else should have heard about (i.e. the awesome talk you will be giving soon)! It's really up to you!