Safer attachment removal

• Confirm before deleting. Removing an attachment from a ticket now asks you to confirm first, and lets you know once it's gone.
• It's on the record. Adding or removing an attachment is now logged in the ticket's Activity tab, so the team can see who added or removed what, and when.

Also fixed: avatars and images could show as broken after a while because a browser held on to an expired link. They now refresh as they should.