Route prefix is not supported

[mkahvi]

For example:

GET https://host/systems/alienrpg/images/IndBackground3.png 403 (Forbidden)

When it should be trying to access

https://host/myRoutePrefix/systems/alienrpg/images/IndBackground3.png

Problem is caused by starting paths with slash, forcing them to exit the route prefix scope.

So the asset path is defined as /systems/alienrpg/images/IndBackground3.png
When it should be systems/alienrpg/images/IndBackground3.png
Though this one since this is in CSS, it needs to be relative to the CSS file

Tested on version 3.1.8 since I can't update to Foundry v12 yet.