TracAdmin allows attachment to be added to non-existent resource

[Ryan J Ollos]

To reproduce:

• trac-admin $ENV attachment add wiki:SomeNonExistentPage file
• Navigate to /attachment/wiki/SomeNonExistentPage and the attachment will be found
• Navigate to /wiki/SomeNonExistentPage to find the following:

Even worse, an attachment could be created for a non-existent realm:

$ trac-admin $ENV attachment add some:none file1

When attempting to view the attachment at the path /attachment/some/none/file1, the permission check would fail: ATTACHMENT_VIEW privileges are required to perform this operation on Attachment 'file1' in some:none. You don't have the required permissions.. However, inspection of the database or execution of the trac-admin attachment list command shows that the attachment is present.

Proposed changes can be found in log:rjollos.git:t11397. There is one additional changes proposed:

• Raise an error if navigating to an attachment page for which the parent resource doesn't exist.