Add login/logout to the frontend about quickfeed

[s111]

I've updated the backend to redirect to /app/logout on logout and /app/newlogin on a new login. The constants can be changed in web/auth/auth.go if needed.

• On navigating to /app/newlogin the user's data should be fetched, and the URL should probably be rewritten to /app/home, immediately.
• On navigating to /app/logout a You've been logged out message (or similar) should be shown to the user.

[s111]

I've updated the /auth endpoints to take a ?redirect=url query parameter. To keep the current behavior the urls needs to be changed to:

• /auth/:provider?redirect=/app/newlogin
• /logout?redirect=/app/logout