Modify the 'No results' message from trash_flag to be Open Atrium specific.

This depends on this patch to trash_flag to add a 'No results' message in the first place:

https://www.drupal.org/node/2298879

Basically, seeing the "Archive" page empty with just a title is very confusing to new users, since they don't know what is *supposed* to be there. This gives them an idea!