feat(auth): add configurable base url

[axiomofjoy]

Adds PHOENIX_BASE_URL parameter. Similar to root_url in Grafana.

The URL used to access Phoenix from a browser. This is needed to ensure that the
`redirect_uri` parameter is correct during OAuth2 authorization code flows if an
OAuth2 IDP is configured. If you have a reverse proxy in front of Phoenix that
exposes Phoenix through a subpath, ensure that the subpath appears at the end of
this URL. Strongly consider using HTTPS since HTTP is insecure.

resolves #4690