Add GET /dashboards to return a list of dashboards

This endpoint returns a list of all dashboards in the database, formatted with:

- `id`, the UUID of the dashboard
- `title`
- `url`, the Stagecraft endpoint for the dashboard
- `public-url`, the dashboard's location on GOV.UK
- `published`, a boolean indicating publishedness
Works for all the dashboards we have in production... would you recommend pagination?