refs https://github.com/TryGhost/Team/issues/1104
- bumped `@tryghost/custom-theme-settings-service` so it throws a more appropriate `ValidationError` when setting keys don't exist or a select value is not known
- changed the custom theme settings service to have a `.init()` method which creates an instance of the service under `.api` so that we're able to create the instance at a particular point in the boot process when we know the models have been initialised
- there were problems in tests because the service was being initialised through the require chain before models were initialised through the boot process
- fixed incorrect `camelCase` of resource name in API responses
refs https://github.com/TryGhost/Team/issues/1070
- added `@tryghost/custom-theme-settings-service` as a dependency
- `core/server/services/custom-theme-settings` creates an instance of the new service passing in the model used for storing the setting keys/values and a cache instance
- requiring `core/shared/services/custom-theme-settings-cache` creates a cache instance, it has no dependencies so can be required anywhere and the first require will initialize the shared instance
- updated the theme activation bridge to trigger the theme settings service to sync the newly activated theme settings and populate the cache
- updated theme validation to pass `labs` through as an option so that we get custom theme settings back as part of the checked theme as that's what is passed to the custom theme settings service
