Fixed `cache.populate()` leaving behind old settings
Fixed `cache.populate()` leaving behind old settings

refs https://github.com/TryGhost/Team/issues/1070

- if the cache is repopulated, keys that already existed in the cache but that don't exist in the new group of settings were left in the cache
- added a `.clear()` method that removes all items from the cache and call it when populating so the cache only contains what it was last populated with
  - deletes properties on the internal content object so that references aren't lost

TryGhost/Ghost,TryGhost/Ghost,TryGhost/Ghost