smarter validators on data mapping tab

[bzkrouse]

Current behavior is to outline the input in red and block saving if there is an empty input.

In the future, it would be nice to distinguish true requirements from optional (e.g. is it a problem if the sorting column is empty?) and only do the blocking/red coloring if there is really a requirement that needs to be met. Otherwise, just drop any empty inputs when the save button is pressed.