Updated refresh Icon
Wrote a session handler

- logs users out after a period of inactivity
- redirects users to log-in page if attempting to visit a page which
needs sign-in
Replaced all database_lecname with unitcode

- also slimmed down delete_unit code
- added session destroy to log out
Adding units and displaying quiz questions

- adding/deleting units works properly
- lecturers can only see their own quiz questions now
Rewrote studview_unit and tweaked other files
Fixed delete_student code for single database