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

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

• ?labels=None: Return results where there are no labels.
• ?labels=none: Return results where there are no labels.
• ?labels=Any: Return results where that is at least one label.
• ?labels=any: Return results where that is at least one label.

Applies to these API endpoints:

• List project issues
• List group issues
• List dashboard issues (i.e. https://docs.gitlab.com/ee/api/issues.html#list-issues)
• List project merge requests
• List group merge requests
• List dashboard merge requests