Fix related documents functionality and implementation.

[smithellis]

Adding related documents to an article is broken in several ways. This case unifies those issues to be addressed at the same time.

See 2259, 2260, 2261

[smithellis]

mozilla/kitsune#6610

[smithellis]

I cleaned this PR up a bit - I'd left some artifacts from adding restriction status to the search results, which turned out to be out of scope/not needed for the case.

[akatsoulas]

We should have fixed each one of the linked issues individually which would have made testing and reviewing easier. Since we already have a single PR for all 3 issues, let's work with it to avoid extra work by splitting it to 3 pieces. With that said, I am closing this ticket and we can work with the 3 individuals. This way we can test each one and submit fixes if needed individually. Otherwise this ticket won't be resolved unless the other 3 are resolved. I believe it also adds confusion whether the 3 tickets are similar/same or not.