Expose the milestone ID in the UI

What does this MR do?

Exposes the milestone ID in the UI.

Closes #328419 (closed).

Screenshots (strongly suggested)

The only issue is that the sidebar will end up with two identical copy button icons when collapsed. Edit: we're actually doing the same in MRs where we have the reference and the branch name

Does this MR meet the acceptance criteria?

Conformity

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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
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Security

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