Adding the ApplicationUserManager and ApplicationSignInManager Classes

In this step, we'll be adding two new classes to our project: ApplicationUserManager and ApplicationSignInManager. Notice that both class names use the prefix "Application" to disambiguate these classes from the Identity classes that there'll be inheriting from.

Add a "Security" folder to the root of the Treehouse.FitnessFrog.Shared class library project. Then add the `ApplicationUserM...