Implement `Fully Specced Out` Extended Merge Request Popover

Follow up issue to https://gitlab.com/gitlab-org/gitlab-ce/issues/54916

Fully specced out	Ready to merge	Barebones	Skeleton loading

In initial pass, we only did the Barebones version of this. There is some work to be done to add statistics and creator/assignee to the bottom

This design adds the following:

• Assignee
• Milestone
• Tasks completed/total
• MR Health
  • Originally from https://gitlab.com/gitlab-org/gitlab-ce/issues/47007, this gives a good idea of how far towards being merged a MR is.
  • We wouldn't be showing extra tooltips on top of a tooltip though.
  •