Add Thong as backend maintainer for gitlab-ce|ee

Trainee issue: #3357 (closed)

• I have been a team member for about 11 months, and merged about 139 MRs in that time.
  • This includes a Kubernetes development guide
  • I also help maintain Auto-DevOps.gitlab-ci.yml
• Prior to joining GitLab, I have some familiarity with the GitLab codebase since 8.X - mainly in the authentication and SSH area
• I have been a reviewer for gitlab-ce|ee for about 8 months
• As trainee_maintainer for 6 months, I have reviewed at least 46 merge requests in various areas:
  • API Performance - #3357 (comment 148194064)
  • SSO change - #3357 (comment 172023680)
  • Prometheus for group clusters - #3357 (comment 167299256)
  • Rename UrlValidator - #3357 (comment 159712607)
  • Sentry fix - #3357 (comment 155854590)
  • and other security MRs in dev.gitlab.org

That's the pros. In terms of weakness, I would love to spend more time reviewing MRs with authorization problems (e.g. getting this right in API is sometimes not easy).

WDYT @gitlab-org/maintainers/rails-backend ?