Use correct link for exhibition/event visual stories in all content search

[davidpmccormick]

What

We don't get the related exhibition/event in the content API when we get a visual story, so we aren't able to build the URL that we want to display the visual stories at (/exhibitions/{exhibition_uid}/visual-story). They do still work at the generic visual story route (/visual-stories/{visual-story-uid}) so it's not currently a big problem, but would be good to fix. If we want to do this before we change the url structure in any way, it would require a change to the Content API.