Guiding principle: put as much in the URL as is reasonable.
JSON <---> HTTP
HTTP Requests:
Structure of URL:
/home/username/worksheet_num/cell_id/...
- ../update
JSON Messages:
Database Calls:
- increase_worksheet_state_number()
- update_cell_input(cell_id, input_text)
- evaluate_cell(cell_id)
- insert_after_cell(cell_id)