GitLab for Slack app - Re-enabling for groups does not reenabling for projects

Problem

From #461056 (comment 1917668874):

It seems like enabling the integration at the top-level group will only allow underlying projects to successfully use it the first time you enable it. If at any point you disable it then inheritance doesn't work anymore if you attempt to re-enable it, steps to reproduce:

1. Enable the Slack App integration at the top-level group, by linking the group to the Slack workspace.
2. Confirm all projects under it can use the integration (send notifications, etc.).
3. Disable the integration at the top-level group by unlinking the Slack workspace from the group.
4. Confirm that the integration no longer works on underlying projects (expected).
5. Re-enable the integration at the top-level group, again by linking the group to the Slack workspace.
6. Confirm that even though the integration becomes enabled in the underlying projects, the link between the project and Slack does not get inherited and thus the integration does not work (unexpected).

Projects would have to be individually linked to the Slack workspace which is unpractical.

Proposed solution

See #468593 (comment 1963888834).