Make notification branch selection a concern

Signed-off-by: Balasankar "Balu" C <balasankar@gitlab.com>

app/models/project_services/notification_branch_selection.rb → app/models/concerns/notification_branch_selection.rb

 # frozen_string_literal: true
 
-# This module will be included in ChatNotificationService and
-# PipelinesEmailService classes. Not to be used directly.
+# Concern handling functionality around deciding whether to send notification
+# for activities on a specified branch or not. Will be included in
+# ChatNotificationService and PipelinesEmailService classes.
 module NotificationBranchSelection
+  extend ActiveSupport::Concern
+
   BRANCH_CHOICES = [
     [_('All branches'), 'all'],
     [_('Default branch'), 'default'],