Auto-delete files with zero links

If deleting a message including a file, the file won't vanish from filesystem (in GridFS not noticeable, as you don't access it normally), even if the message is the last occurrence of a link to the file. It does no longer show up in attachment overview on the right bar, too.
So an "easy" check for number of links to a file when deleting a message would save space (especially for AWS) and may reduce problems (which may occur if uploads are unprotected or perhaps someone stored the link to the file, which shouldn't be posted/uploaded).

The issue concerns message deletion and the attachment overview; start by tracing how deleting a message handles its linked files and how the overview determines remaining links. Done means a file with no remaining message links is removed from storage without breaking the attachment overview, including the GridFS case.