[Feature flag] elastic: Enable "start indexing" button

What

The web interface for indexing is hidden behind a feature flag. Now the indexing functionality is stable, we can remove the feature flag and make the interface available to public.

Also related https://gitlab.com/gitlab-org/gitlab-ce/issues/64074

Owners

• Team: ~"group::editor"
• Most appropriate slack channel to reach out to: #g_create_editor
• Best individual to reach out to: @toupeira

Expectations

What are we expecting to happen?

• For gitlab.com: not much, we don't plan to use it in the near future
• For self-managed instances: admins will start using the button

What might happen if this goes wrong?

If the admins don't read the docs, they might miss that they have to manually create the ES index through a Rake task.

This is being discussed at https://gitlab.com/gitlab-org/gitlab-ee/issues/13717

What can we monitor to detect problems with this?

• For gitlab.com:
  • https://dashboards.gitlab.net/d/9GOIu9Siz/sidekiq-stats
  • https://dashboards.gitlab.net/d/000000124/sidekiq-workers
• For self-managed instances:
  • user reports through ZenDesk or GitLab

Roll Out Steps

• Enable on staging
• Test on staging
• Ensure that documentation has been updated → https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31972
• [-] Enable on GitLab.com for individual groups/projects listed above and verify behaviour
• [-] 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 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