Prevent custom warning messages for global rules

This prevents an abuse filter admin from setting a custom warning
message (which are references to a non-translated, local wiki page),
for global filters (which can be on any wiki in any language).

bug: 45164
Change-Id: If61716053b75f45e4b70f78d3809ddd02ac6d83e
Merge "Prevent custom warning messages for global rules"