Prevent any new Slack Notifications integrations from being created on GitLab.com

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Proposal

Now that GitLab.com users can use the GitLab for Slack app for Slack notifications, as part of &8670 (closed), we want to prevent any new Slack Notifications integrations from being created by users of GitLab.com only, but still allow existing Slack Notifications integrations to be edited.

This issue is blocked by:

• People need to be able to configure the GitLab for Slack app at group and instance-level #391526 (closed).
• We need to allow people to easily migrate their existing settings from the Slack notifications integration to the GitLab for Slack app integration #372412.

Technical implementation

These tasks are for backend:

1. Hide the Slack Notifications integration from the list of integrations to choose from for users of GitLab.com only (Gitlab.com?), unless they have one created already.
2. Return an error status code in the API if the PUT /projects/:id/integrations/slack call were to create the integration for users of GitLab.com only (Gitlab.com?). (We would allow calls to the endpoint that update existing records).