Clean up code

- Remove unneeded requires
- Use fork URL for comments

app/models/project_services/chat_message/base_message.rb

 # frozen_string_literal: true
 
-require 'slack-messenger'
-
 module ChatMessage
   class BaseMessage
     RELATIVE_LINK_REGEX = /!\[[^\]]*\]\((\/uploads\/[^\)]*)\)/.freeze

app/models/project_services/chat_message/pipeline_message.rb

 # frozen_string_literal: true
-require 'slack-messenger'
 
 module ChatMessage
   class PipelineMessage < BaseMessage

app/models/project_services/slack_service.rb

@@ -36,7 +36,7 @@ class SlackService < ChatNotificationService
     private
 
     def notify(message, opts)
-      # See https://github.com/stevenosloan/slack-notifier#custom-http-client
+      # See https://gitlab.com/gitlab-org/slack-notifier/#custom-http-client
       notifier = Slack::Messenger.new(webhook, opts.merge(http_client: HTTPClient))
 
       notifier.ping(