WIP
Generating thumb for ptiff images
Skeleton for API endpoint for requesting thumbnail images
Showing correct thumbnails from the session list template
Using thumb endpoint for /sessions template
Merge branch 'dev/dj-thumbnails'

* dev/dj-thumbnails:
  Using thumb endpoint for /sessions template
  Showing correct thumbnails from the session list template
  Skeleton for API endpoint for requesting thumbnail images
  Generating thumb for ptiff images
  WIP