Q&A Sort: Display first level of responses to responders.

In use of Q&A sort, we've seen that community response to an
answerer is an important part of the experience. This shows
the first level of the responses to the OP, but keeps the rest
collapsed for brevity.

Author: umbrae

r2/r2/models/builder.py

@@ -983,25 +983,30 @@ def _make_wrapped_tree(self):
             comment.num_children = num_children[comment._id]
             comment.edits_visible = self.edits_visible
 
+            parent = wrapped_by_id.get(comment.parent_id)
+
             # In the Q&A sort type, we want to collapse all comments other than
             # those that are:
             #
             # 1. Top-level comments,
             # 2. Responses from the OP(s),
-            # 3. Responded to by the OP(s) (dealt with below), or
-            # 4. Otherwise normally prevented from collapse (eg distinguished
+            # 3. Responded to by the OP(s) (dealt with below),
+            # 4. Within one level of an OP reply, or
+            # 5. Otherwise normally prevented from collapse (eg distinguished
             #    comments).
             if (qa_sort_hiding and
-                   depth[comment._id] != 0 and # (1)
-                   comment.author_id not in special_responder_ids and # (2)
-                   not comment.prevent_collapse): # (4)
+                    depth[comment._id] != 0 and  # (1)
+                    comment.author_id not in special_responder_ids and  # (2)
+                    not (parent and
+                         parent.author_id in special_responder_ids and
+                         feature.is_enabled('qa_show_replies')) and  # (4)
+                    not comment.prevent_collapse):  # (5)
                 comment.hidden = True
 
             if comment.collapsed and comment._id in dont_collapse:
                 comment.collapsed = False
                 comment.hidden = False
 
-            parent = wrapped_by_id.get(comment.parent_id)
             if parent:
                 if (qa_sort_hiding and
                         comment.author_id in special_responder_ids):