office-ui-fabric-react v6.166.0

Minor changes

• These changes allow for a controlled Pivot to have no selected PivotItem by specifying null as the selectedKey prop. (commit by nico-bellante, pr #8660)

Patches

• ThemeGeneratorPage: fix it so it looks right (commit by joschect, pr #8686)
• Updating CalloutContentBase's shouldComponentUpdate to do shallow comparision (commit by whykushal93, pr #8688)