R1:f9836cb646f8

Scramble file secrets when related objects change policies

Summary:
Ref T10262. Files have an internal secret key which is partially used to control access to them, and determines part of the URL you need to access them. Scramble (regenerate) the secret when:

- the view policy for the file itself changes (and the new policy is not "public" or "all users"); or
- the view policy or space for an object the file is attached to changes (and the file policy is not "public" or "all users").

This basically…