Set correct Grafana URL

Summary:
The parameters domain and root_url are useful when behind a reverse proxy
to indicate Grafana how to serve URL like redirections.

For example, when asking to generate a short URL:

• without any configuration, it uses socket://localhost:3000/goto/
• with domain set, it uses socket://grafana.nasqueron.org:3000/goto/
• with root_url set it correctly uses https://grafana.nasqueron.org/goto/

References:

• https://grafana.com/docs/grafana/latest/setup-grafana/configure-grafana/
• https://grafana.com/tutorials/run-grafana-behind-a-proxy/

Ref T651

Test Plan: Ask to share a dashboard, then shorten URL

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T651

Differential Revision: https://devcentral.nasqueron.org/D3380