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
