Clear markdown preview when toggling back to Edit

This fixes the issue with previewing empty markdown (though maybe we
should really prevent that), as well as making sure they don't get a
misleading obsolete preview if they lose internet or something similar.

tildes/static/js/behaviors/markdown-edit-tab.js

@@ -8,5 +8,6 @@ $.onmount('[data-js-markdown-edit-tab]', function() {
 
         $editTextarea.removeClass('d-none');
         $previewDiv.addClass('d-none');
+        $previewDiv.empty();
     });
 });