Allow Hiding/Collapsing of Milestone header on Roadmap

Problem to solve

As a Roadmaps user, I'd like to hide/collapse the milestone header, so I can keep the roadmap clean when presenting it to stakeholders who aren't concerned about individual time boxes.

Intended users

• Rachel (Release Manager)
• Parker (Product Manager)
• Delaney (Development Team Lead)

Proposal

• A click to collapse option for the milestone header
• Add milestone icon with total milestone count to milestone row
• Milestones are open by default

Specs: https://gitlab-org.gitlab.io/gitlab-design/hosted/alexis/%23212494-collapse-roadmap-milestones-spec-previews/

Documentation

Update roadmap documentation

Availability & Testing

What does success look like, and how can we measure that?

Monitor Roadmap view SMAU