Split out Swagger UI into separate container

[rjw57]

The PI dashboard datamart API also provides a swagger UI via a separate container configured in the docker-compose.yml file. Rather than having the webapp serve it's own Swagger UI, we should just use the official container.

The container allows configuration of OAuth2 client via the OAUTH2_CLIENT_ID environment variable and we should update our served swagger.json to have the appropriate OAuth2 token URLs taken from the settings.