Fix for #162 - Relative urls for static content

[jdhollander]

This is a fix for issue #162.

Fixes regression where relative urls for the static content became absolute, thus breaking reverse proxy usage. This commit reverts to relative urls for this content, taking into account whether the route that delivers them has a trailing slash or not.

I have tested this behind a reverse proxy, albeit not in great detail. Prefixing the path as outlined in the regression report works perfectly.

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[mcollina]

lgtm