Notification: Tests and Implementations

Closes #12 (closed)

Changes in AppUser.java:

• Added new field called notificationList and unreadNotifCount
• Added methods addNotification, resetUnreadNotifCount, and getNotifications

NotificationServiceImpl:

• getUserNotification
• userCheckNotification

Notification.java:

• compareTo compares the instance's createdDate with another instance's createdDate

NotifType:

• java enum with constants: PROJECT and SYSTEM
• might get updated

Navbar:

• added link to navigate to Notifications (only for logged in users)