Any logged in user can delete any attachments

The fix for bug 48771 allows any logged in user to delete any attachment.

Tom suggests:
"I think this should be restricted to allowing any logged in user to delete their own attachments, or any member of the Launchpad Admins team to delete any attachment."

Followed by Bjorn:
"I guess that's quite sensible, although I think bug contacts should be
able to delete attachments as well. The main reason for that is that
theres a re-tracing service for Ubuntu's crash reports, which should be
able to delete the attached core dumps. Until we have a proper crash
database, it needs to have permission to delete other people's
attachments."