View group milestones on dashboard milestone page

Description

• View group milestones on dashboard milestone page, i.e. it would appear here for the gitlab.com instance: https://gitlab.com/dashboard/milestones.
• Use the same UI as the group milestones page, where you have both project milestones and group milestones shown. An example of that group milestones page for the gitlab-org group is https://gitlab.com/groups/gitlab-org/milestones.

Original description

Summary

Browsing to /dashboard/milestones and the new Group Milestones aren't listed

Steps to reproduce

Create a Group Milestones and browse to /dashboard/milestones

What is the current bug behavior?

Group Milestones are missing

What is the expected correct behavior?

Group Milestones should be listed