Add popover for iteration details

!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. A similar popover should be added for iterations to convey additional details; this will be especially useful once we move to using iteration names in place of dates as the reference.

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

Contexts to add popover (current hover state)

Edited by Kushal Pandya