[react-62] Set Navbar so that it changes depending on whether or not user is logged in #63
Description
The Navbar should check to see whether or not a userId exists (subject to change), and if it doesn't exist, then the options to Login and Register will show on the Navbar, which will link to the login and register pages.
If the userId does exist, then there should be a logout button, where it should link them to the login page (also subject to change).