Use milestone popover for milestone field values

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!147797 (merged) introduces a milestone popover used for GFM milestone references. This popover contains information such as the dates and progress on the milestone which provide additional context.

This context is equally relevant from lists, boards, and sidebar details.

Contexts to add popover (current hover state)

• GFM links
• Work item sidebar (no hover state)
• Incident sidebar (no hover state)
• Child items
• Issue list (tooltip)
• Issue board (tooltip w/ no date)
• Issue dashboard (tooltip)
• MR list (tooltip)
• MR dashboard (tooltip)