Filter by `None`/`Any` for milestone in issues/mrs/boards
See &454 for filtering on different fields in different scenarios.
Milestone: None: Return results where there is no milestone selected.
Milestone: Any: Return results where that is a milestone selected.
- Project issue list filter
- Group issue lists filter
- Project merge request list filter
- Group merge request list filter
- Project board filter
- Group board filter
The four selections from the filter should be: