Purge all relevant Slack data when Slack app is uninstalled

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This issue came out of this discussion #384226 (comment 1241527987).

When the GitLab for Slack app is completely uninstalled from a workspace we should purge all related Slack-related data (ChatName, SlackIntegration at time of writing).

We would subscribe to the app_uninstalled event https://api.slack.com/events/app_uninstalled and handle it within Integrations::SlackEventService.

This request was also made by the Slack app Review team on 24 Jan 2023:

It looks like when the app is currently uninstalled, the connection still remains within the web app. I believe this can be handled by subscribing to the app_uninstalled event, mentioned here: https://api.slack.com/events/app_uninstalled.

Availability & Testing

Risks include

• Accidentally purging a ChatName for a different workspace
• Accidentally leaving orphaned project integrations

Suggestions:

• feature tests that mock the Slack subscription and validate that the above do not happen.