refs https://github.com/TryGhost/Team/issues/1097
- added `customThemeSettingKeys` as an argument to `preview.handle()` because we can't know which keys should be allowed through up-front
- added `custom` as a supported setting in the preview header data
- `custom` should be a JSON object containing any custom theme settings
- we parse the object but only set properties on `@custom` that are known custom theme setting keys
- if parsing fails or it's not an object then no custom data is set
- updated `updateLocalTemplateOptions()` to pull `.custom` off of the preview data and pass it through so it's accessible on `@custom` as an override to the saved custom data
refs https://github.com/TryGhost/Team/issues/1097
globalTemplateOptions are supposed to be static with localTemplateOptions being merged in per-request, however the per-request preview data was being extracted and set in the global options. Comments suggest that the global data should be static and eventually updated via other means, the usage of the request object to get per-request preview data is working against that.
- adjusted the preview handler to return an object rather than changing properties by reference on a passed in object
- moved preview data fetching out of `getSiteData()` used in `updateGlobalTemplateOptions()` and into `updateLocalTemplateOptions()` so that we're not relying on the request object in `updateGlobalTemplateOptions()`
