Allows user to delete account

* Adds delete button to user show page
* Adds confirm message on submit
* Adds destroy action to users_controller
* Adds :delete resource to routes for users
So, this is better as you've secured it to the point where you can only see the "Delete Account" button if you are looking at yourself.

However, if I was a malicious user, I could still manually make a `DELETE` request to say `/users/10` and successfully delete that user myself. I would add some kind of check in your controller to ensure that the user can only delete themselves.