Getting started - Better empty state for Milestones view
### Description
Milestones are awesome. Their empty state is not. Currently the empty state for the Milestones view is literally a blank page. We can do better.
### Specification
* The same empty state should be used for `Open`, `Closed` and `All` milestones.
### Wireframe
### Links / references
* Part of gitlab-ce#15632
* Issue similar to gitlab-ce#20853
issue