Implement OAuth2 Authentication

Closes #451 (closed) #452 (closed) #453 (closed) #458 (closed)

• The grader table has been modified to include oauth_id and set it as the primary key instead of the name
• Accessing any endpoint redirects to /oauth/start to initiate login with the OAuth provider. Once a user is logged in, login_user is called which allows access to Zesje and redirects to /
• /oauth/callback is the callback endpoint which should be called by the third party provider
• /oauth/grader gives the current logged in grader's details
• On the frontend, users are redirected to Login.jsx whenever the server throws a 401
• The grader dropdown list has been removed. Graders are set based on current user who is logged in.
• A mock oauth2 server has been set up in the development environment.