Add a "spam" column to the bad header and banned files quarantines

[rjl]

Items that contain invalid mail headers or banned file attachments are quite often spam, but the current quarantine pages for these item types only offer the user the choice between releasing the item or deleting it. It should be possible to add a third column that would re-classify the item as confirmed spam.

[dmorton]

The code I did recently to unify the various caches takes care of this.