closes https://github.com/TryGhost/Team/issues/1310
- added `/setup/finishing-touches` route
- refreshes theme preview on access
- uses existing setting form components to build a reduced settings menu
- updated `/?firstStart=true` handling to transition to `/setup/finishing-touches` instead of showing the get-started modal
- updated standard setup flow to show the get-started modal upon completion
- `/?firstStart=true` flow for now will only be used when a theme has been chosen before site creation
