add responsive CSS to hyperlinked images when thumbnail caption markup is used

When thumbnail caption markup is used in combination with a hyperlinked image, the image is not responsive. (non-hyperlinked images are fine, like the example in the docs) Adding a selector to apply responsiveness to hyperlinked images when thumbnail captions are used.
Merge pull request #10662 from kevinlisota/thumbnail-images

add responsive CSS to hyperlinked images when thumbnail caption markup used