Before this commit you would have to do this in multilingual setups:
```
---
title: "Custom!"
url: "/jp/custom/foo"
---
```
This commit allows for relative URLs, e.g:
```
---
title: "Custom!"
url: "custom/foo"
---
```
Which is obviously easier and more portable.
The meaning of relative may change to include more in the future (e.g. role based access).
Fixes#5704