CONN-435: make cacheable GET requests for RelatedForumDiscussion

Make cacheable GET requests to RelatedForumDiscussionController instead of POST.

* force JSON format on the backend side, to have consistent URLs (in AJAX requests and those pushed to purger queue)
* cache on backend for 6 hours, no caching on client-side

This change will remove a huge part of 10mm requests backend receives daily.

@nandy-andy / @RafLeszczynski / @themech / @prein / @michalroszka