JavaScript enhancements for the FormsetDataTable

This is optional enhancements of how FormsetDataTable is displayed when
there is an extra empty row. It removes the extra row, adds a button
that lets you add as many extra empty rows as needed, and replaces the
"delete" checkboxes with × icons that also hide the deleted row
immediately.

Change-Id: I7e10d5695127a4f45175911b326dca1c266a2fd5
Merge "JavaScript enhancements for the FormsetDataTable"