Resolve "29 - Create User (and children) classes"

Ticket: #29 (closed)

Description

Created the AttendeeUser and OrganizerUser header files and class implementations. Also did some folder restructuring.
Use g++ backend/src/lib/user_attendee.cpp backend/src/lib/user_organizer.cpp backend/src/test/test_user.cpp -o test_user.out -I backend/include/ to compile the tests from the repository directory.

TODO

• Create AttendeeUser header file.
• Implement AttendeeUser class and methods.
• Create OrganizerUser header file.
• Implement OrganizerUser class and methods.
• Peer review and merge