Removed registration functionallity and fixed an issue with login form

Now the login form shows an error when the authentication fails

	modified:   PCnstein/urls.py
	modified:   PCnsteinapp/templates/base.html
	modified:   PCnsteinapp/templates/mainpage.html
	modified:   PCnsteinapp/templates/registration/login.html
	modified:   PCnsteinapp/views.py