Fix GroupActivityCard relative metric links

What does this MR do and why?

Account for a custom relative_url_root when generating metric links on the Group page.

How to set up and validate locally

1. Set relative_url_root to /path:

gdk config set relative_url_root /path
gdk reconfigure
gdk restart

1. Visit any group page: http://gdk.test:3000/path/flightjs
2. Validate that the metric links are working as expected

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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

Related to #411315 (closed)