[Feature flag] Roll-out custom emoji
## What
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/37911 introduces the `:custom_emoji` feature flag.
This is part of the roadmap laid out in https://gitlab.com/groups/gitlab-org/-/epics/1820
This feature flag should be enabled when a viable minimal product has been reached according to the roadmap at https://gitlab.com/groups/gitlab-org/-/epics/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?
<!-- Which dashboards from https://dashboards.gitlab.net are most relevant? -->
## 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
- [x] Enable on staging
- [x] Test on staging
- [ ] Ensure that documentation has been updated
- [x] 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
- [x] 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](https://docs.gitlab.com/ee/development/feature_flags/controls.html#where-to-run-commands)) and in your team channel
- [x] 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](https://docs.gitlab.com/ee/development/feature_flags/controls.html#cleaning-up) by running chatops command in `#production` channel
issue