refs https://github.com/TryGhost/Team/issues/1107
- updated schema validation to allow `'image'` through as a known setting type now that Admin has support
- added transformation of setting values for `'image'` types because they will be URLs and should be stored with `__GHOST_URL__`
refs https://github.com/TryGhost/Team/issues/1106
- updated schema validation to add `'boolean'` as an allowed `type` value
- added `format()` and `parse()` methods to `CustomThemeSetting` model to match `Settings` model behaviour for boolean-type settings
refs https://github.com/TryGhost/Team/issues/1070
- stores values of custom theme settings
- will be merged with full settings data parsed from themes for API output
- will be cached and made available for lookup in themes to avoid db roundtrips
- stores type of custom theme settings so we can coerce values and know if the type has changed when syncing
- records will be synced with themes upon activation
