refs https://github.com/TryGhost/Team/issues/1106
- removed `'string'` from the `attr()` definition for `customThemeSetting.value` so that we're not coercing the value from the API into something that doesn't work when applied to the `checked` attribute on checkbox inputs
- added initial `<CustomThemeSettings::Boolean>` component and used it for display in theme setting forms when `type === 'boolean'`
