Move SlackService::Notifier module into file

The module is also used in MattermostService
to make it more clear that it is not a SlackService
internal. This commit moves it into its own file

app/models/project_services/mattermost_service.rb

 # frozen_string_literal: true
 
 class MattermostService < ChatNotificationService
-  include ::SlackService::Notifier
+  include SlackMattermost::Notifier
 
   def title
     'Mattermost notifications'

app/models/project_services/slack_mattermost/notifier.rb

+# frozen_string_literal: true
+
+module SlackMattermost
+  module Notifier
+    private
+
+    def notify(message, opts)
+      # See https://gitlab.com/gitlab-org/slack-notifier/#custom-http-client
+      notifier = Slack::Messenger.new(webhook, opts.merge(http_client: HTTPClient))
+      notifier.ping(
+        message.pretext,
+        attachments: message.attachments,
+        fallback: message.fallback
+      )
+    end
+
+    class HTTPClient
+      def self.post(uri, params = {})
+        params.delete(:http_options) # these are internal to the client and we do not want them
+        Gitlab::HTTP.post(uri, body: params)
+      end
+    end
+  end
+end

app/models/project_services/slack_service.rb

 # frozen_string_literal: true
 
 class SlackService < ChatNotificationService
+  include SlackMattermost::Notifier
+
   prop_accessor EVENT_CHANNEL['alert']
 
   def title
@@ -35,27 +37,4 @@ class SlackService < ChatNotificationService
 
     super
   end
-
-  module Notifier
-    private
-
-    def notify(message, opts)
-      # See https://gitlab.com/gitlab-org/slack-notifier/#custom-http-client
-      notifier = Slack::Messenger.new(webhook, opts.merge(http_client: HTTPClient))
-      notifier.ping(
-        message.pretext,
-        attachments: message.attachments,
-        fallback: message.fallback
-      )
-    end
-
-    class HTTPClient
-      def self.post(uri, params = {})
-        params.delete(:http_options) # these are internal to the client and we do not want them
-        Gitlab::HTTP.post(uri, body: params)
-      end
-    end
-  end
-
-  include Notifier
 end

spec/support/shared_examples/models/slack_mattermost_notifications_shared_examples.rb

@@ -7,7 +7,7 @@ RSpec.shared_examples 'slack or mattermost notifications' do |service_name|
   let(:webhook_url) { 'https://example.gitlab.com' }
 
   def execute_with_options(options)
-    receive(:new).with(webhook_url, options.merge(http_client: SlackService::Notifier::HTTPClient))
+    receive(:new).with(webhook_url, options.merge(http_client: SlackMattermost::Notifier::HTTPClient))
      .and_return(double(:slack_service).as_null_object)
   end