Merge branch 'add-title-images' into 'master'

Add title images for news entries

See merge request hifis/hifis.net!83

_posts/2020/08/2020-08-03-HZI.md

 ---
 title: "HZI-HIFIS collaboration for Coronavirus Surveys"
-title_image: default 
+title_image: mitchell-luo-H3htK85wwnU-unsplash.jpg
 data: 2020-08-03
 authors:
   - jandt

_posts/2020/09/2020-09-01-VPN.md

 ---
 title: "Start of Helmholtz VPN"
-title_image: default
+title_image: omar-flores-MOO6k3RaiwE-unsplash.jpg
 data: 2020-09-01
 authors:
   - jandt

_posts/2020/09/2020-09-16-hifis-homepage-helpdesk.md

 ---
 title: "HIFIS Pages and Helpdesk launched"
-title_image: default
+title_image: spacex-uj3hvdfQujI-unsplash.jpg
 data: 2020-09-16
 authors:
   - jandt