Add controller and views for group & instance GitLab for Slack app

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/142868



Merged-by: Justin Ho Tuan Duong <hduong@gitlab.com>
Approved-by: Rajan Mistry <rmistry@gitlab.com>
Approved-by: Sarah Yasonik <syasonik@gitlab.com>
Approved-by: Justin Ho Tuan Duong <hduong@gitlab.com>
Reviewed-by: Sarah Yasonik <syasonik@gitlab.com>
Reviewed-by: Justin Ho Tuan Duong <hduong@gitlab.com>
Reviewed-by: Bojan Marjanovic <bmarjanovic@gitlab.com>
Co-authored-by: Luke Duncalfe <lduncalfe@eml.cc>