Subgroup links should use the subgroup full path

What does this MR do?

Fixes #208279 (closed) and #205375 (closed)

Links to milestones in subgroups are generated incorrectly in the milestones list. Instead of linking to their own subgroup they link to the top-level group, whether or not a milestone with that iid actually exists there. If it doesn't, the result is a 404. If it does, the link takes you to that one instead.

Actually the second case is more destructive because the close/reopen buttons also point to this milestone. So in trying to close the subgroup milestone the user will unexpectedly close a different one.

Screenshots

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

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• 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
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