Refactoring the Menu Into a Partial View

To make it more apparent when a user is signed-in or not, we'll update the header of our web app to display the signed-in user's username—which in our case, is the user's email address.

> Ideally, the user account information would also include their first and last name so we could display one or both of those values instead of their email address. But since we haven't extended the user acco...