Link new issue to original via checkbox

What does this MR do and why?

When using the issue header action to create a new issue, !68226 (merged) added the text "Related to #." to the new issue's description.

This improves the UX of that change by:

• Renaming the "New issue" menu item to "New related issue" to clarify that it will create a link.
• Replacing the added description text with a new checkbox widget, to avoid obliterating the issue template.

I've split this change off from !76895 (closed), based on UX feedback from @hollyreynolds and @uhlexsis and @engwan.

How to set up and validate locally, with screenshots

1. Check out branch

2. Go to http://127.0.0.1:3000/Commit451/lab-coat/-/issues/1

3. At the top of the issue, click the issue header actions dropdown menu — notice that the link text has changed from "New issue" to "New related issue"
   

4. Click "New related issue"

5. Notice that the URL includes a new issue[add_related_issue] parameter
   

6. Notice that the new issue form now has a checkbox labeled "Relate to issue #1"
   
   This new checkbox only appears when the issue[add_related_issue] parameter is present (so it doesn't clutter up the issue form otherwise).

7. Click "Create issue"

8. Notice that the newly-created issue is linked to the original issue
   

MR acceptance checklist

• I have evaluated the MR acceptance checklist for this MR.

Closes #198494 (closed)