* [Issue #27602] Added the concept of an invalid file placement rule.
- If a custom file placement rule no longer has any of its disks checked, then it's considered invalid.
- Invalid rules will be marked with a red file placement icon and an exclamation mark on the file placement's rules tab.
An appropriate tooltip will be shown as well.
- Invalid rules have no effect on file placement.
- Folder rules can never be invalid (they will be automatically cleaned up as before.)