TASK-05-2 Refactoring: use numeric user-id
TASK-05-2 Refactoring: use numeric user-id

Since we implemented an FK-relation on the users and notes tables, we should
save a numeric user-id instead of a string when inserting a note.
Inserting a note also returns the new note's id.

MIMUC-MMN/assignments-ws-15-16,MIMUC-MMN/assignments-ws-15-16,MIMUC-MMN/assignments-ws-15-16