[Discuss] Implement curated groups for threads (such as "Hot", "New" and "Unanswered")

[galadhremmin]

This would be helpful to further drive engagement on the website. There are currently no groups that are curated by filters, so this would be net new capabilities in addition to the hand-crafted groups we've got today (such as "General conversation" and "Profiles.")

Ideas for useful categories:

• Hot - threads seeing unusual engagement, perhaps the topic is controversial or of interest to the community.
• New - new threads for regular viewers who would like to view what was posted since they last visited.
• Unanswered - threads that haven't seen any engagement at all.