Fix display thumbnails

Something happened to this function along the way and it stopped displaying thumbnails. `display_post_thumbnail_src()` was creating an array of urls to thumbnails in the post, and `display_post_thumbnail()` had a mysterious `$src` param with a value that could never be equivalent to the output of `display_post_thumbnail_src()`. So it displayed nothing. We just need one image url or the `DEFAULT_PHOTO` constant returned from `display_post_thumbnail_src()`, and `display_post_thumbnail()` just needs to check if `display_post_thumbnail_src()` has a value and display it along with the right class. I think we were trying to be too fancy.