Fix VSD drilldown links with a custom relative_url_root

Mentioned in thread: !122099 (comment 1412171344)

The drill-down links in the Value Stream Dashboard are broken when a custom relative_url_root is being used.

Steps to Reproduce

1. gdk config set relative_url_root /path
2. gdk reconfigure
3. gdk restart
4. Visit any value streams dashboard with table content
5. Click any metric name in the left-most column
6. Instead of seeing the detailed metric view, you will get a 404