Enable testability of GitLab for Slack app notifications

What does this MR do and why?

This enables the testability of the GitLab for Slack app notifications feature.

It will let the integration admin know if there are channels configured that GitLab cannot post to, and suggest to them that if the channel is private to add the GitLab Slack app to the channel first.

#383894

Screenshots or screen recordings

How to set up and validate locally

To QA this feature you must have access to the Ecosystem Test Slack Workspace.

1. Start a new GitPod instance and configure the GitLab for Slack (dev) app to work with it (docs)
2. Checkout this feature branch, bundle and yarn, and restart GDK (docs):

    cd /workspace/gitlab &&
        git fetch origin 383894-test-gitlab-for-slack-app &&
        git checkout 383894-test-gitlab-for-slack-app &&
        bundle &&
        yarn &&
        cd /workspace/gitlab-development-kit &&
        gdk restart

3. Enable the flag. On the GitPod terminal: cd /workspace/gitlab && bundle exec rails c, then:

   Feature.enable(:integration_slack_app_notifications)

4. Choose a project, and set up the Slack application integration.
5. Once set up, you can specify a channel to post events to.
6. Choose a public channel, and a private channel.
7. Click Test.
8. You should see a toast about the inability to post to the private channel.
9. To post to a private channel, go to the private channel, type /add and choose "Add apps to this channel", then choose GitLab (dev).
10. Test again, it should be successful.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #383894