Fix request for group labels with a subgroup

What does this MR do?

When selecting a subgroup, in value stream analytics, the request for group labels includes the subgroup in the path, causing a 404 error

This MR:

• Updates the grouplabels request to use a parent_id if one exists, otherwise the full_path
• Provides the parent_id to the vue app on load if we have a subgroup
• Additional specs to cover creating and editing a custom stage for a subgroup

Screenshots

No ui changes

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team