Resolve "Simplify milestone summary"

What does this MR do?

Moves all of the milestone summary content into a sidebar similar to the issues and merge request pages.

Prerequisite MRs:
gitlab-ce!10163
gitlab-ce!10224

EE Port:
gitlab-ee!1511

Are there points in the code the reviewer needs to double check?

Ensure the milestone page functions correctly in each of the following paths

• /[group]/[project]/milestones/[id]
• /[group]/milestones/[id]
• /[dashboard]/milestones/[id]

Screenshots (if relevant)

Project Milestone	Project Milestone (collapsed)

Dashboard Milestone	Group Milestone

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [ ] Documentation created/updated
• [ ] API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #23674 (closed)