office-ui-fabric-react v7.18.0

Minor changes

• Shimmer: refactor out the BaseComponent and deprecated React lifecycle methods. (commit by Vitalius1, PR #9922)

Patches

• Fix cell animations so they only trigger on cell key changes (commit)
• Panel: fix shrinking of the footer and make header not shrink. (commit)
• Pivot: align pivot link text in relation to the whole pivot tab. (commit by Vitalius1, PR #9951)