Fix possible race condition when processing statuses (mastodon#10815)

Author: ClearlyClaire

app/lib/activitypub/activity/create.rb

@@ -267,7 +267,11 @@ def fetch_replies(status)
   def conversation_from_uri(uri)
     return nil if uri.nil?
     return Conversation.find_by(id: OStatus::TagManager.instance.unique_tag_to_local_id(uri, 'Conversation')) if OStatus::TagManager.instance.local_id?(uri)
-    Conversation.find_by(uri: uri) || Conversation.create(uri: uri)
+    begin
+      Conversation.find_or_create_by!(uri: uri)
+    rescue ActiveRecord::RecordInvalid, ActiveRecord::RecordNotUnique
+      retry
+    end
   end
 
   def visibility_from_audience