Deduplicate unique accounts and limit account array in descendants CTE

Limit unique accounts to check blocks from to 30 unique accounts (any new
participant will have its blocks ignored for the purpose of displaying the
posts). This is extremely unlikely to be reached in a legitimate situation,
but would limit wasteful computation in specially-crafted threads with many
participants.

app/models/concerns/status/threading_concern.rb

@@ -84,7 +84,11 @@ def descendant_ids(limit, depth)
       UNION ALL
 
         SELECT
-          statuses.id, path || statuses.id, authors || statuses.account_id
+          statuses.id, path || statuses.id, (CASE
+            WHEN array_length(authors, 1) >= 30 THEN authors
+            WHEN statuses.account_id = ANY(authors) THEN authors
+            ELSE authors || statuses.account_id
+          END)
         FROM
           search_tree
         JOIN