[Feature flag] Roll-out custom emoji

What

!37911 (merged) introduces the :custom_emoji feature flag.

This is part of the roadmap laid out in &1820

This feature flag should be enabled when a viable minimal product has been reached according to the roadmap at &1820

Owners

• Team: Community contribution
• Most appropriate slack channel to reach out to: #mr-coaching
• Best individual to reach out to: @toon

@raju249 is doing most of the work to implement the feature. @toon is acting as the counterpart on the GitLab team to help out with this rollout.

Expectations

### What are we expecting to happen?

What might happen if this goes wrong?

What can we monitor to detect problems with this?

Beta groups/projects

If applicable, any groups/projects that are happy to have this feature turned on early. Some organizations may wish to test big changes they are interested in with a small subset of users ahead of time for example.

• gitlab-org/gitlab project
• gitlab-org/gitlab-com groups
• ...

Roll Out Steps

• Enable on staging
• Test on staging
• Ensure that documentation has been updated
• Enable on GitLab.com for individual groups/projects listed above and verify behaviour
• Coordinate a time to enable the flag with #production and #g_delivery on slack.
• Announce on the issue an estimated time this will be enabled on GitLab.com
• Enable on GitLab.com by running chatops command in #production
• Cross post chatops slack command to #support_gitlab-com (more guidance when this is necessary in the dev docs) and in your team channel
• Announce on the issue that the flag has been enabled
• Remove feature flag and add changelog entry
• After the flag removal is deployed, clean up the feature flag by running chatops command in #production channel