Comment popovers from markdown

What does this MR do and why?

MVC for Add comment summary to the link's tooltip when ... (#29663)

If you link to a comment, the popover now shows the comment text instead of the issue title.

Makes it easier to quickly reference which comment someone is referring to, useful for long discussions.

Behind nearby_comment_tooltips feature flag.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Feature.enable(:nearby_comment_tooltips)
2. Visit an issue page, add a comment with a link to another comment
3. Hover