Take advantage of AuthenticationResult

[pbrisbin]

• Modify authenticateUser to return an AuthenticationResult
• Internally, use Either to propagate error messages that will ultimately get logged in the case of failed authentications
• Specifically parse credsExtra based on the plugin used
• Separate plugin-specific behavior in implementation and test

Reasoning:

Using AuthenticationResult and returning useful ServerError values, should help debugging. Splitting out the credsExtra parsing this way will making adding new plugins (that return different credsExtra maps) easier. Supporting the dummy plugin via dummyProfile makes it possible to write plugin-agnostic examples in the test and only test plugin-specific behavior in the GitHub (and eventually Google, Twitter, etc) contexts.

[JoelQ]

LGTM :+1:

[pbrisbin]

Addressed all comments, rebased, will merge on Green. Thanks!