Add Authentication APIs

[polastre]

Define, create tests, and implement APIs and an API scope for a front-end server to implement authentication (username/password check, or OAuth login).

[polastre]

I created an "authentication flow" diagram for the OAuth-specific use case. Here's the current version of the diagram, and a link to the Google Drawing for those within GSA.

https://docs.google.com/a/gsa.gov/drawings/d/1TxL6jiywFq1FGesH13TGtKdKr9KIeI7FBM1FgLVvuJg/edit?usp=sharing