Milestone tab badges

What does this MR do?

Adds count badges to the tabs on the Milestone page to mirror the Issue page.

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

CSS: Padding was added to the .milestone element to make expired milestones look good.

Why was this MR needed?

For consistency with other tabs

What are the relevant issue numbers?

Closes #20114 (closed)

Screenshots (if relevant)

Before

After

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together