Fix "ok" button issue.

In some configurations of OS'es and browsers (Linux, Chrome/Chromium),
buttons for submitting user input after dropping an item don't render
correctly; the "ok" label is broken up into two lines:

******         *****
* ok *   vs.   * o *
******         * k *
               *****

The changes in this commit fix this issue.

drag_and_drop_v2/public/css/drag_and_drop.css

@@ -111,6 +111,7 @@
     position: absolute;
     left: 150px;
     top: 4px;
+    white-space: nowrap;  /* Fix cross-browser issue: Without this declaration, button text wraps in Chrome/Chromium */
 }
 
 .xblock--drag-and-drop .drag-container .option .numerical-input.correct .input-submit,