Broken group milestone links

Summary

The group milestone overview includes the milestones of subgroups, but the link does refer to the current group instead of the subgroup where the milestone resides.

Steps to reproduce

1. Create a group and subgroup.
2. Create a milestone in the subgroup
3. In the parent group go to issues > milestones and try to click on the milestone

What is the current bug behavior?

The milestone link results in a 404 and the subgroup milestones are only reachable from that subgroup itself.

What is the expected correct behavior?

The subgroup milestone can be reached from the parent groups milestones view.