Margin on form-items in table rows (Classy)

Classy puts a margin on form items, but not when they're in table rows.

This causes problems in contrib: if forms are placed inside responsive tables, there is no spacing between fields:
#2668708: Spacing between fields on paras →

This behavior dates back to Drupal 4, and the reasoning is lost in the mists of time. Probably it is that core sometimes has a single form element in a table row (like a checkbox or dropdown) and no more spacing is desired than would be needed for text alone.

The least disruptive solution is:
When a table row has a single form item prevent it from adding unnecessary extra spacing, but if it has multiple form items allow spacing between them.