Make user popovers accessible via keyboard

What does this MR do and why?

Make user popovers accessible

JS based solution to move focus to popover and back.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Go to an issue with some comments, assignees and threads and tab though, focus on user popovers should move within popover (e.g. for Follow/Unfollow button)

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #513105 (closed)

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/511714, https://gitlab.com/gitlab-org/gitlab/-/issues/511665, https://gitlab.com/gitlab-org/gitlab/-/issues/511655, https://gitlab.com/gitlab-org/gitlab/-/issues/511375