Ability to edit debouncing rules in brave://adblock

[fmarier]

brave://adblock/ has a field for custom filter rules which is really handy when working with @ryanbr to get an existing rule fixed or a new ad blocked.

It would be really helpful to have the same functionality to inject custom entries to the debouncing rule set so that new rules could be easily tested prior to shipping them to users.

Specifically, we need to be able to:

1. Add a new rule.
2. Remove an existing rule (perhaps by overriding an existing pattern with a do-nothing action).