[#971] Updated organisation main page

- The organisation main page now has more live data
- A change in 'img.html' makes sure that (see organisation directory):
  - The thumbnails are 'PNG' so that transparent backgrounds don't become black
  - The thumbnails are not cropped, but instead a margin is applied, to keep correct ratio
[#971] Removed unnecessary 'thumbnail' import
Merge pull request #975 from akvo/971_organisation_page

[#971] Updated organisation main page