When CDN is enabled / disabled at runtime, some stale entries are not correctly revved in cache. Not 100% sure what
causes this, but it's suspected to have something to do with static transformation, as it only ends up being CSS
files that still have CDN references left hanging around in them after CDN is disabled. At any rate, this class will
listen for CDN dark features being enabled / disabled, and will obliterate the webresources cache to be safe.
We also clear the cache on the ConfluenceCDNStrategyFactory.