Refactor the WebCompat interventions code to re-use the shims code

[wisniewskit]

Issue #248

• to reduce friction when creating interventions, it would be good to have one "type" of intervention, sharing the same single codebase/JSON file.
• right now we have three: shims, UA overrides, and interventions. we can just make them all shims with some kind of "type" field.
• this would consolidate the code, making it easier to know how to add a given intervention, and to fix some bugs.
• this would also give us a chance to improve test coverage.
• rough time estimate: one week of engineering time for the core refactor, and likely another week to improve test coverage.