Implement IDP functionality

[florian-jaeger]

Is your feature request related to a problem? Please describe.
The grader service should be able to be a identity provider. For example the JupyterHub should be able to authenticate against the Grader Service.

[adammccartney]

Can you please make sure to document the proposed changes? We would like to build up a set of design documents for grader service in the docs/ folder. They don't need to be long, possibly using the following headings as a template - there are a couple of dumb examples under each heading, you can choose more appropriate ones!

General Scope

• how do the authentication and authorization flows currently work? where does the user authenticate? Who then authorizes access for the user?
• how do the proposed changes differ from the current implementation?
• why the proposed changes are an improvement

Use cases

• what are the types of access that we are trying to control?
• who is going to be accessing what?

Requirements

• exactly what will you be implementing - there should be specific classes and functions to cover the use cases

Schedule

• how long will it take you?

[adammccartney]

Reminder to consider #79 when looking at a solution for this