app assets are not accessible behind nginx proxy on kubernetes

[Similar issue on another pulsar-dashboard](https://github.com/bbonnin/pulsar-express/issues/19).

Pulsar-manager web dashboard never loads up when we host pulsar-manager behind an LB/Proxy service with a url slug.

It looks like all the static contents are loaded from the absolute path instead of the relative path.

Start by reproducing the dashboard behind an LB or proxy service with a URL slug, then inspect how static asset URLs are generated. The issue is done when the Pulsar Manager web dashboard loads successfully and its static contents are requested through the relative proxied path.

Written by the indexing model from the issue text.