FIX: Make sure featured category topics are unique (#18343)

In the past, CategoryFeaturedTopic.feature_topics raised an exception
sometimes because it tried to create multiple CategoryFeaturedTopic
records for the same topic.

This code should not raise any exceptions as long as the list of new
topic IDs is unique because all previous records are deleted first,
then recreated and everything happens inside a transaction. The previous
rescue block was dead code anyway because it tried to catch
PG::UniqueViolation instead of ActiveRecord::RecordNotUnique. This
commit includes a fix to ensure that the topic IDs are unique.

app/models/category_featured_topic.rb

@@ -60,24 +60,19 @@ class CategoryFeaturedTopic < ActiveRecord::Base
 
     # Add topics, even if they're in secured categories or invisible
     query = TopicQuery.new(Discourse.system_user, query_opts)
-    results = query.list_category_topic_ids(c).uniq
+    results = query.list_category_topic_ids(c)
 
     # Add some topics that are visible to everyone:
     anon_query = TopicQuery.new(nil, query_opts.merge(except_topic_ids: [c.topic_id] + results))
-    results += anon_query.list_category_topic_ids(c).uniq
+    results += anon_query.list_category_topic_ids(c)
 
+    results.uniq!
     return if results == existing
 
     CategoryFeaturedTopic.transaction do
       CategoryFeaturedTopic.where(category_id: c.id).delete_all
-      if results
       results.each_with_index do |topic_id, idx|
-          begin
         c.category_featured_topics.create(topic_id: topic_id, rank: idx)
-          rescue PG::UniqueViolation
-            # If another process features this topic, just ignore it
-          end
-        end
       end
     end
   end