Resolve "Implement backend Parent-Child relation for feedback"

Closes #552 (closed)

Changes:
feedback.py

• get: Added parent parameter and children parameter in the returned object. (children is a list of FO ids)
• post: Added parent as an argument added to the created feedback option
• put: Added parent as an argument in the returned data (parent cannot be modified).

database.py

• FeedbackOption: Add 'parent_id' column (type:int (fk feedback option), can be nullable)
• FeedbackOption: Add 'children' relationship (self-referential one-to-many relationship)

test_feedback.py
Add 7 tests:

• Create a FO without a parent
• Create a FO with a parent
• Delete a FO without a parent
• Delete a FO with a parent
• Delete a FO with 1 child
• Delete a FO with 2 levels of children (children and sub-children)
• Get the children of a FO