"Create auto-categorization rule" does not work if label contains a slash

Due to the way the URL is built, if a label contains a slash, the button will redirect to the wrong URL.

See https://framagit.org/kresusapp/kresus/-/blob/main/client/components/rules/urls.tsx#L13

STR:

1. Create a transaction with a title "Test with / slash"
2. Click on the "Create auto-categorization rule" button