Disallow presigned urls based on cache headers #469

[bwalkerl]

Test instructions:

1. Setup objectfs and enable Pre-Signed URLs.
2. Upload a file with https://github.com/catalyst/moodle-tool_publicfiles or find any other immutable public files.
3. Start by enabling the setting and confirm Pre-Signing is used on the file (partly to confirm Pre-Signing is working)
4. Disable the new setting
5. Open dev tools and disable caching as the Pre-Signed URL will still likely be cached.
6. With dev tools still open load the file and confirm the file isn't redirected to the Pre-Signed URL.

Closes #469