feat(persistent-onboarding): Add renderAvatar property to ContentPreview

[robertjk]

In Persistent Onboarding project I need to add a tooltip over Avatar component inside Preview. For that to work I defined renderAvatar property, which can be passed down in EUA and wrap this component in a tooltip. If renderAvatar is not defined Avatar will render normally, so the old code will still work as it used to.

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[ConradJChan]

Changes look good to me, but there is an outstanding question from @jstoffan. Also, are there any unit tests that need to be updated as well?