REST API support for GitLab for Slack app integration

About

The GitLab for Slack app integration (model: Integration::GitlabSlackApplication) is not supported by the REST API.

The lack of support was mostly likely intention due to the integration requiring setup via OAuth with Slack to be enabled, and that it historically had no other settings to configure. We've subsequently added features to this integration like support for GitLab event notifications to be posted to Slack channels, meaning API endpoints would now have more value.

Proposal

The REST API should support:

• update
• disable

but not enable. This is because enabling requires a person to do the OAuth dance between GitLab and Slack, where the user agrees to the requested scopes on a Slack web server.