Fix "ok" button issue.
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.

edx-solutions/xblock-drag-and-drop-v2,edx-solutions/xblock-drag-and-drop-v2,arbrandes/xblock-drag-and-drop-v2,open-craft/xblock-drag-and-drop-v2,arbrandes/xblock-drag-and-drop-v2,edx-solutions/xblock-drag-and-drop-v2,arbrandes/xblock-drag-and-drop-v2,edx-solutions/xblock-drag-and-drop-v2,open-craft/xblock-drag-and-drop-v2,open-craft/xblock-drag-and-drop-v2,open-craft/xblock-drag-and-drop-v2,arbrandes/xblock-drag-and-drop-v2