Feature: Add authentication flow & tests

[derekherman]

Summary

This pull request adds the completed authentication flow with full test coverage and is intended to be merged into the 1.0-release-prep branch before it gets merged.

Coverage goes us to 93.13% with this PR and all linting passes.

Implements #108

Checklist

• [x] My pull request is addressing an open issue (please create one otherwise).
• [x] My code is tested and passes existing tests.
• [x] My code follows the Contributing Guidelines (updates are often made to the guidelines, check it out periodically).

[spacedmonkey]

This PR looks great!

There are a couple of nit picks, but nothing that blocks this PR from being merge. Just things we may want to look at later.