Custom URL with slash makes the item unaccessible

[olli-gold]

Describe the bug
When an item has a custom URL with a slash inside (for example "slash/example"), the item cannot be displayed any longer on the frontend. When you try to access such an item, you'll just get an error 404. It's also impossible to edit the item and to get into the administrative options - so it's very hard (or even impossible) to remove the custom URL on the frontend.

To Reproduce
Steps to reproduce the behavior:

1. Create a custom URL with a slash for an item
2. Get this item in a browser
3. You will probably see a 404 page

Expected behavior
The item should be displayed normally

Related work
I'm not aware of any related work about that