new placeholder thumbnails for projects and groups

* add (public domain) placeholder thumbnails for groups and projects
 - projects: http://thenounproject.com/term/gears/1063/
 - groups: http://thenounproject.com/term/group/15061/
* use new placeholder thumbnails instead of holder.js
* add layout for group/project index pages to reduce code duplication
* remove the no longer used holder.js