Show answer for imageresponse

[BLD-21](https://edx-wiki.atlassian.net/browse/BLD-21)

"Show answer" on imageresponse should highlight the area that would have earned full credit if the student had left her marker there.

Example of XML:
```xml
<problem>
  <imageresponse>
    <imageinput src="https://studio.edx.org/c4x/edX/DemoX/asset/Dog-and-Cat.jpg" width="640" height="400" rectangle="(385,98)-(600,337);(185,18)-(400,120)" regions="[[[10,10], [20,30], [40, 10]], [[100,100], [120,130], [110,150], [110,350], [50,50]]]"/>
  </imageresponse>
  <solution>
    <div class="detailed-solution">
      <p>Explanation</p>
      <p>The animal on the right is a kitten. The animal on the left is a puppy, not a kitten.</p>
    </div>
  </solution>
</problem>
```
@jmclaus and @auraz please review.