Merge pull request #568 from simong/568-share-feed

Groups who are private and joinable show up in share feed, yet you cannot share with them
Better image preview processor

 * Thumbnail and wide images are converted to JPG (transparent pixels are set to white)
 * GIF images are converted to JPG
 * Other images retain their original format
Fixed bug in storing principal profile picture. We use  rather than  for multipart file objects.
Made convertToJPG an ImageUtil utility.
It is now being used for both content preview thumbnails as profile pictures.

When the user uploads his profilepicture, it will be converted to a plain JPG.