Break timestamp_cache_key out into method on model
Alias "application/vnd.contentful.management.v1+json" as json. This actually causes a warning because a constant is being overwritten. Don't know if there's a better way (it must be json)
Working webhooks controller, which requires an accepts just the params for the content type ID and entry ID.

At the moment this only clears timestamp caches for entries. Probably enough.