Let administrators restrict users' ability to adjust filter settings by mail type

[rjl]

Maia already lets administrators control whether users can enable/disable filtering by mail type, but this control needs to extend to the settings themselves, so that users can be prevented from changing the destiny and/or thresholds associated with a given mail type.

Richard Whittaker wanted, for instance, to prevent users from selecting the "Labeled" delivery option for viruses, preferring to limit them to "Quarantined" or "Discarded". With this enhancement the system and domain defaults for viruses would be inherited by users and would not be alterable except by administrators.