Add confirmation modal when deleting system label
**This is expected to become a no longer be a problem with the [introduction of workspaces](https://gitlab.com/groups/gitlab-org/-/epics/4257). As a result, it was decided to close for now.**
In https://gitlab.com/gitlab-org/gitlab-ce/issues/48456 we applied the same UI/UX of the **project/group labels** pages to the **system labels** page.
One of the things missing is to add the modal to confirm label deletion:

`[label] will be permanently deleted and will not be added to new projects. Existing projects will not be affected. This cannot be undone.`
issue