v2.28.0

Changed

• Disable button to sign a contract in learner dashboard
  if the order is not validated

Fixed

• useDateFormat did not return undefined with null values
• PurchaseButton instantiated a hook conditionnaly
• Make UserHelper manages OpenEdxProfile object to extract name
• Do not display unpublished children categories in the public
  category detail page

Added

• Load React JS bundles from CDN on all pages