closes https://github.com/TryGhost/Team/issues/1491
With multiple newsletters feature, a site should always have at-least one newsletter by default. Also, as with the default product, the default newsletter also needs to be renamed to the site title during the setup flow.
- adds default newsletter to main and test fixtures
- updates setup flow to rename newsletter name and sender name to site title
- updates model to extend default value for fields
- updates test
