This adds a new secure_uploads_pm_only site setting. When secure_uploads
is true with this setting, only uploads created in PMs will be marked
secure; no uploads in secure categories will be marked as secure, and
the login_required site setting has no bearing on upload security
either.
This is meant to be a stopgap solution to prevent secure uploads
in a single place (private messages) for sensitive admin data exports.
Ideally we would want a more comprehensive way of saying that certain
upload types get secured which is a hybrid/mixed mode secure uploads,
but for now this will do the trick.
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