Tested account login and account creation

Note: had to remove the primary key property on UserID/QuizID (MySQL
throws an error if an unlimited TEXT column is set as a primary key).
Account login and account creation now works well. HomepageServlet
redirects to login page if no User-property is found in the session. I
also added a small test servlet for displaying messages to the user.

TODO today: List available quizzes, create quiz summary, and let users
take/create quizzes.