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
refs https://github.com/TryGhost/Team/issues/1070
- having a dependency on a model in the cache service meant that Ghost had to know about that and pre-initialize the cache during boot, even though that didn't actually do anything except create a cache instance
- by making the cache a simple key/value store able to be populated from the cache settings service when a theme is activated it means that Ghost doesn't need to perform any extra initialization when the cache is initialized via `require`
refs https://github.com/TryGhost/Team/issues/1070
- added `bread` util that acts as a wrapper for the provided model, if we have any business functionality needed when settings are added/removed then it will go here
- added primary "server" service that handles syncing of custom theme data extracted from a theme with the settings that are in the database and exported as "Service". Syncing rules on theme activation:
- if a new setting is seen, create it with the default value
- if a setting has it's type changed, remove it and create a new setting with the default value
- if a select setting's value is not a valid option, reset it to the default value
- added shared "frontend/server" service that exposes an in-memory cache of key/value pairs for the currently active theme
