Make Stan Hu a backend maintainer for CE and EE

As per the process, I created this issue to make a case for becoming a CE and EE maintainer in public, following the informal nomination by @smcgivern in https://gitlab.com/gitlab-org/gitlab-ce/issues/41364#note_106326781.

In the interest of full transparency, here are links to recent MRs that I've reviewed in depth:

• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22372
• https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6456
• https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5532
• https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/7808#note_110148564
• gitlab-org/omnibus-gitlab!2583 (merged)

Admittedly I've merged things that should have been caught by me (e.g. https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21653). I welcome any feedback about improving on reviews.

/cc: @grzesiek @DouweM @rymai @smcgivern @ayufan @rspeicher @dzaporozhets @nick.thomas @tommy.morgan