Port drush commands to D8

Ok so I don't think we need to port over those two commands exactly:

1) cas-user-add-role - There's a drush command for adding roles to users, don't see why we need this.
2) cas-user-create - I think this does too much. Drush already has a command for creating new users. All we need is the ability to associate that user with some CAS username.

I think we're fine adding just one command which associates an existing account with CAS.

Ugh... no idea why those tests are failing.

Trying again after I removed the branch alias from upstream.

oops

These tests pass locally. I'm 95% sure these failures are related to the fact that we're modifying the composer.json file, and it's tripping up the test runner somehow. It seems like it's not pulling down the external auth dependency.