Filter by `None`/`Any` for labels in issues/mrs/boards

See &454 for filtering on different fields in different scenarios.

• Label: None: Return results where there are no labels.
• Label: Any: Return results where that is at least one label.

Applies to:

• Project issue list filter
• Group issue lists filter
• Project merge request list filter
• Group merge request list filter
• Project board filter
• Group board filter