Merge branch '3353-trainee-backend-maintainer-ash-mckenzie-update-codeowners' into 'master'

Add @ashmckenzie to backend CODEOWNERS

Closes gitlab-com/www-gitlab-com#3353

See merge request gitlab-org/gitlab-ce!27204

.gitlab/CODEOWNERS.disabled

 # Backend Maintainers are the default for all ruby files
-*.rb @ayufan @dbalexandre @DouweM @dzaporozhets @godfat @grzesiek @mkozono @nick.thomas @rspeicher @rymai @smcgivern
-*.rake @ayufan @dbalexandre @DouweM @dzaporozhets @godfat @grzesiek @mkozono @nick.thomas @rspeicher @rymai @smcgivern
+*.rb @ashmckenzie @ayufan @dbalexandre @DouweM @dzaporozhets @godfat @grzesiek @mkozono @nick.thomas @rspeicher @rymai @smcgivern
+*.rake @ashmckenzie @ayufan @dbalexandre @DouweM @dzaporozhets @godfat @grzesiek @mkozono @nick.thomas @rspeicher @rymai @smcgivern
 
 # Technical writing team are the default reviewers for everything in `doc/`
 /doc/ @axil @marcia