Track Rollout of Object deduplication for GitLab.com

Object deduplication is regarded a beta quality feature, and GA is the target for %11.11.

This means that we should roll out the feature on gitlab.com to verify the feature works as expected.

Steps involved; (please edit if I'm wrong)

• Verify RC2 contains https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26821
• Wait for rc2 to be deployed to production
• Determine what project to enable the project first for
  • This project needs to use hashed storage
• Enable the feature using Chatops
• Manually fork this project, check if everything worked as expected through prometheus and by other means
• Enable for more project

This issue should be closed by the merge request that removes the feature flag on gitlab-ce.