Complete CRUD for group-level integrations

Description

After adding the CRUD for group-level integrations #209825 (closed), !27557 (merged), we were missing the group_id column #209824 (closed) so an integration cannot belong to a group.

Currently, we use Project.first for testing purposes but we have to fix that. https://gitlab.com/gitlab-org/gitlab/-/blob/v13.2.3-ee/app/controllers/groups/settings/integrations_controller.rb#L16-20

This is under a feature flag group_level_integrations.