Editing Variable-Length Line Item Edits (section)

=== Default selectors ===

* `.line-item`: Row element with the following attributes holding values that are passed to the AJAX scripts:
** `data-id`: Record id of the line item in the database.
** `data-type`: Name of the PHP class that will be instantiated to fetch and save edits to the line item in the AJAX script.
* `.add-line-btn`: Button with click handler to add new line items to the list.
* `.line-item`: Row element with double-click handler to edit existing line items.
* `.save-line-btn`: Button added to the DOM while editing or deleting a line item that commits the changes.
* `.cancel-line-btn`: Button added to the DOM while editing or deleting a line item that cancels the edit.
* `#csrf-token`: Hidden element holding a CSRF token value that is then passed to the AJAX script to prevent cross-site forgery.