Comment popovers for comments on the current page

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 on the current page, 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.

• Doesn't include formatting or markdown features, and only shows the first 100chars of text. It probably should but then it's challenging to get the size and formatting right.
• Shows the normal issue popover if the comment doesn't have text (e.g. image only)

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 a comment from that page
3. Click the invite members button.