Updated database files for peer reviews

createDatabase.php and createDatabaseQueries.txt altered as follows:

- dueDate field added to PEER_REVIEW
- questionType removed from PEER_REVIEW_QUESTION
- reviewID removed from PEER_REVIEW_ANSWER
- ON DELETE CASCADE added to foreign keys in PEER_REVIEW_QUESTION and
PEER_REVIEW_ANSWER

DB_ERD.pdf altered to reflect changes. Couldn't alter Visio file due to
incompatibility.