Use OAuth-based RBAC auth

[Gordonei]

For now, basic auth is fine, but at some point in the future we might want to join Web 2.0 and use OAuth, as vended by our GitLab instance (which in turn might bridge to the City's Active Directory). The big win here would be the different roles, e.g. admin, viewer, allowing people to look at the WUI without breaking anything.

This change is a bit more involved than it would first, as turning on RBAC actually uses a whole different WUI frontend:

1. We would need to wrangle the relevant dependencies, etc.
2. We would also need to mount a new config for the FAB frontend somehow.

Useful links:

• Relevant bit from Airflow docs
• Relevant bit from Flask AppBuilder docs
• Helpful issue from Puckel airflow-docker Repo