FEATURE: allow shadowing all s3 settings

Previously enable s3 uploads and s3 upload bucket were not shadowed.

This caused confusion when people were configuring stuff via env cause most
of s3 settings are shadowed.

config/site_settings.yml

@@ -1074,6 +1074,7 @@ files:
   enable_s3_uploads:
     default: false
     client: true
+    shadowed_by_global: true
   s3_use_iam_profile: false
   s3_access_key_id:
     default: ""
@@ -1090,6 +1091,7 @@ files:
   s3_upload_bucket:
     default: ""
     regex: '^[a-z0-9\-\/_]+$' # can't use '.' when using HTTPS
+    shadowed_by_global: true
   s3_endpoint:
     default: ""
     regex: '^https?:\/\/.+[^\/]$'