files: Require authentication to view attachments

This feature adds a setting to the control panel to require signing in to
view attachments. This is in addition to the security already provided in
the download URLs. Currently, download URLs are signed for a specific help
desk, and automatically expire after about 24 hours. The exact timing is the
following midnight allowing for at least 12 hours cache time.

Administrators can impose this extra security feature to refuse serving
attachment files if the user is not currently signed in. This could prevent
third-party users from viewing an attachment if they were able to get access
to the download URL before it expired.