Destroy method with admin and proper user authentication.

@maneyt , @katherineabuhadal 

Key changes:

-Added destroy in routes, controller, and view

-Application controller and user model have authentication methods to access destroy

-Removed flash messages from application controller since no legit user would get those errors anyway

-Removed admin? method from user model because all columns in the database with a boolean datatype automatically come with method_name? for free. In other words, admin? method already is available to us automatically upon creation. 
This `before_action` is only going to be used in the `PostsController`, so I'd move it there.