FIX: Don't rescue `PG::UniqueViolation` within a transaction.

Also acquire a transaction per link instead of failing when
any of the links can't be processed.

This prevents ActiveRecord from rolling back the transaction
and the next SQL statement sent to PG will fail. This is
however hard to test as it only happens when there are
two competing process trying to process this method at the
same time.

app/models/topic_link.rb

@@ -111,9 +111,6 @@ SQL
   def self.extract_from(post)
     return if post.blank? || post.whisper?
 
-    added_urls = []
-    TopicLink.transaction do
-
     added_urls = []
     reflected_ids = []
 
@@ -130,6 +127,8 @@ SQL
       .reject { |_, p| p.nil? || "mailto".freeze == p.scheme }
       .uniq { |_, p| p }
       .each do |link, parsed|
+
+      TopicLink.transaction do
         begin
           url = link.url
           internal = false
@@ -185,7 +184,7 @@ SQL
                                 link_post_id: reflected_post.try(:id),
                                 quote: link.is_quote,
                                 extension: file_extension)
-            rescue ActiveRecord::RecordNotUnique, PG::UniqueViolation
+            rescue ActiveRecord::RecordNotUnique
               # it's fine
             end
           end