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.
Merge pull request #242 from alphagov/77610398-add-get-list-dashboards

Add GET /dashboards to return a list of dashboards