feat(Checkbox): add asFlexbox prop to allow for different layout styles

[eszthoff]

When using checkboxes in LabelPicker in browsers that show scroll bars in the traditional way (not as overlays) the layout breaks and labels are displayed in a separate line from the checkbox itself. In order to fix this, we need the checkbox to be in a flexbox. However, this as default behaviour can negatively effect already existing implementations. Hence this option is added via a new prop.

This new prop is now used in LabelPicker.

This PR also fixes a small bug with passing style and className to Checkbox that were applied to the wrong HTML element.

Checklist

• The implementation has been manually tested and complies with Textkernel browser support guidelines
• The implementation complies with accessibility standards.
• The component has a displayName defined.
• The component comes with a detailed PropTypes (and defaultProps) definition.
• Component PropTypes are sufficiently described / documented.
• There is a story in Storybook.