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/system/s3_uploads_spec.rb

@@ -5,10 +5,9 @@ describe "Uploading files in the composer to S3", type: :system do
 
   let(:modal) { PageObjects::Modals::Base.new }
   let(:composer) { PageObjects::Components::Composer.new }
+  let(:topic) { PageObjects::Pages::Topic.new }
 
   describe "direct S3 uploads" do
-    before { SiteSetting.enable_direct_s3_uploads = true }
-
     describe "single part uploads" do
       it "uploads custom avatars to S3" do
         skip_unless_s3_system_specs_enabled!
@@ -43,7 +42,7 @@ describe "Uploading files in the composer to S3", type: :system do
         setup_s3_system_test
         sign_in(current_user)
 
-        visit "/new-topic"
+        topic.open_new_topic
 
         file_path = file_from_fixtures("logo.png", "images").path
         attach_file(file_path) { composer.click_toolbar_button("upload") }