Only send feedback content to client that should be displayed

In MCQ and MRQ, pop ups could be displayed, with their content, even for items where the user isn't supposed to know if the answer is right or wrong (ie, there is no cross/checkmark icon displayed).

Besides not displaying the feedback popups when the checkmark/cross aren't displayed next to an element, the content of these feedback popups shouldn't be sent to the client at all unless it is meant to be displayed -- otherwise the user can also cheat by looking at the ajax request payload. This prevents the data from being sent in the first place.

@dragonfi This is ready for review - thanks!