Hide dropbox when user with content leave site

This patch will set the hidden attribute to the (not empty) dropbox folder of a user that is not on the site anymore. When the same user comes back the folder is unhidden.

It makes the dropbox folder list more pleasant to read when you can see inactive/hidden folders for users no longer on the site.

The patch is from our 2.8.2 but I have manually applied the changes to trunk to make a clean patch.