FEATURE: Secure uploads in PMs only (#23398)

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.

spec/lib/upload_security_spec.rb

@@ -28,6 +28,14 @@ RSpec.describe UploadSecurity do
           expect(security.should_be_secure?).to eq(true)
         end
 
+        context "if secure_uploads_pm_only" do
+          before { SiteSetting.secure_uploads_pm_only = true }
+
+          it "returns false" do
+            expect(security.should_be_secure?).to eq(false)
+          end
+        end
+
         context "when uploading in public context" do
           describe "for a public type badge_image" do
             let(:type) { "badge_image" }