Highlighting on active page in navigation does not work when running Graylog with a pathprefix.

## Expected Behavior

Usually the active page should be highlighted this way:

It does not work when running graylog with a path prefix. Example path `/graylog/system/configurations`.
With a path prefix the navigation item looks like this when you are currently on the configurations page:

## Your Environment

* Graylog Version: `6.1`