Add image attribute to Dataset and Group

[rgrp]

Add image attribute for Dataset and Group which would be shown in prominent place.

For dataset this could be a relevant image and for Group this would be move of a logo.

I think this has very high value from a UX point of view.

Issues

• Do we need to support storing the image (and resizing etc)?
  • Could use something like transloadit service
  • Use our own storage facility with a reserved directory

[dread]

CMAP are really keen on having this for groups

[dread]

CMAP also interested in having some custom CSS for a group, but that is perhaps another ticket.