LevelBuilder support for play lab

[Finishes: #79639696, #79639780, #79639834, #79639722]
See the "Gotcha" section on the [ActionView::Helpers::FormOptionsHelper docs page](http://api.rubyonrails.org/classes/ActionView/Helpers/FormOptionsHelper.html).  Basically when no option is selected the HTML spec says to send no value to the server.

So we can't get notified that the user deselected all options (where the previous revision had 1 or more options selected) unless Rails includes the hidden form.