After a user has been authenticated at an OAuthProvider's web page, the provider will redirect the browser to the provided redirectUrl. The provider will add a query parameter to the redirectUrl called "code" that represent the authorization code for the user. This method will use the authorization code to validate the user and fetch information about the user from the OAuthProvider. If Synapse can match the user's information to a Synapse user then an access token for the user will be returned. Note: If Synapse cannot match the user's information to an existing Synapse user, then a status code of 404 (not found) will be returned. The user should be prompted to create an account.