Add customizer scope to TeachingBubbleContent

[Vitalius1]

Pull request checklist

• Include a change request file using $ npm run change
  Adding a customizer scope to TeachingBubbleContent to allow pass some fluent styles through Customizer. This issue surfaced when testing Coachmark component example which is using it in its own Callout resulting in a broken fluent style for Coachmark
  

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[Vitalius1]

@micahgodbolt will need your approval for it to be merged.
Also, who could explain how the className registering works in this scenario: We have TeachingBubble registering them once, and then internally it renders TeachingBubbleContent which registers them again. Are we ending up with two classNames with different hashes but with the same styles? Or I am interpreting it wrong 😄

[Vitalius1]

Looks like it's doing what I assumed. The underlined clasess are just a test me applying the same class on both of them and it ends up different classNames with the same styles attached to them. Feels wrong somehow. TeachingBubble registers all of them but uses only root and later TeachingBubbleContent registers them again with different hashes and uses all except the root.

[msft-github-bot]

🎉office-ui-fabric-react@v6.110.2 has been released which incorporates this pull request.:tada:

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