When listDiscussions is called, we need to update sorting by lastViewed

[nedrocks]

listDiscussions query should be updated:

• Grab all discussions that the user should see (as we do today)
• Within this query, ensure to join with the user's viewer object
• Within this query, join with the Posts table grabbing the most recent post for that discussion
• group by:
  • Whether the lastViewedPost timestamp is less than the most recent post
• sort by
  • Most recent post created_at descending.