Consider rebasing related/featured datasets widget on Ajax

[roll]

Overview

For now, we statically populate related/featured datasets for stories/topics (because it had been initially written to handle stories/etc but then used for datasets).

Functionally It works fine having 3000 datasets with slightly slower interface. E.g. https://lacounts-staging.l3.ckan.io/story/edit/a-new-standard-to-help-maintain-la-streets

It could be slower if there are more datasets.

If we have some time I think it's better to rebase on Ajax.