List default rules on user's Subscriptions panel

[Robert Corsaro]

Default rules, configured by the sysadmin, should be listed on the users configuration. They should be read only and always listed as lower priority then the users rules. Because of the implementation of announcer, all rules can't be used as default rules. The rule needs to be able to find it's target subscriber in some sane way. For instance, ticket owner can have a default rule since we can look up the user's name in the ticket change event. General wiki subscriber can not, since we would have to apply it to every user in the system. This would be horrible for performance.

[Robert Corsaro]

(In [9228]) Adds default rules API and user UI

Fixes #7758