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'`
no issue
- custom theme model was missing the `group` attr so the nav menu couldn't see it to assign to groups
- changed homepage group from `home` to `homepage` to match gscan
- added fallback to showing in the "Site-wide" group if an unknown group value is seen