Green List User Management Implementation

[grosscol]

Provide a user management system that only allows a predefined list of users access.

• A UserMgmt sub-class that reads a green list from persistence layer (disk or mongo).
• A way to specify that class is to be used for user management.
• Documentation about where to store the green list.

Stated on a TopMed call, there is a desire to host a closed beta on the public internet with access restricted to a green list of individuals.