WIP: Multiple issue&&MR assignees

• Implement a migration to create table
• Implement a migration to migrate an old data
• Check all migrations on staging
• Fix all the logic around assignee
  • Notifications
    • https://gitlab.com/gitlab-org/gitlab-ee/tree/master/app/services/issuable_base_service.rb#L6
    • https://gitlab.com/gitlab-org/gitlab-ee/tree/master/app/views/notify/_reassigned_issuable_email.text.erb#L6
  • Cache
  • Realations
  • Views
  • Slack commands
  • Fix for elasticsearch