Hide container flex option under containerItemEnabled feature flag

• Type: Task
• Status: Closed
• Priority: Minor
• Resolution: Completed
• Affects Version/s: None
• Fix Version/s: Master
• Component/s: Pages > Page Editor
• Labels:
  • fc_20_06
• Epic Link: Introduce Container fragment allowing Marketers to create reusable fragments visually
• Git Pull Request: https://github.com/liferay-echo/liferay-portal/pull/1275

As discussed with Juan Anton, we need to review how this new feature interacts with existing drag and drop and possible fragment styles.