[Feature flag] Enable and remove `sourcegraph` feature flag

What

Remove the sourcegraph feature flag ...

Owners

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

Expectations

What are we expecting to happen?

• Not much for .com.
• For self-hosted users, they'll have the ability to enable a Sourcegraph integration for their instance.

What might happen if this goes wrong?

• If this feature flag is removed, we will not be able to disable Sourcegraph other than through the admin settings.

What can we monitor to detect problems with this?

• Client-side errors are sent to Sentry.

Beta groups/projects

• gitlab-org/gitlab project
• gitlab-org/gitlab-com groups
• sourcegraph group

Roll Out Steps

• Confirm that QA tests pass with the feature flag enabled (if you're unsure how, contact the relevant stable counterpart in the Quality department)
• Enable on staging (/chatops run feature set feature_name true --staging)
• Test on staging
• Ensure that documentation has been updated
• Enable on GitLab.com for individual groups/projects listed above and verify behaviour (/chatops run feature set --project=gitlab-org/gitlab feature_name true)
• Coordinate a time to enable the flag with the SRE oncall and release managers
  • In #production mention @sre-oncall and @release-managers. Once an SRE on call and Release Manager on call confirm, you can proceed with the rollout
• Announce on the issue an estimated time this will be enabled on GitLab.com
• Enable on GitLab.com by running chatops command in #production (/chatops run feature set feature_name true)
• 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

Rollback Steps

• This feature can be disabled by running the following Chatops command:

/chatops run feature set --project=gitlab-org/gitlab feature_name false

Availability and Testing

Please coordinate the changes to the feature flag status with a SET so that QA team can monitor for any failures.