Retrieve user info from backend

[andrewazores]

In the current graphical user login flow, the user's logged in username is displayed in the masthead user menu but only if using Basic auth, where the user has provided their username directly to the web application. For Bearer auth, ex. in OpenShift, the user supplies a Bearer token, and the application has no username to display. cryostatio/cryostat#704 tracks the backend feature implementing this functionality. Once available, the frontend should be improved to use the backend-supplied user info to populate the masthead user menu.