Guarantee immersive-reader-sdk.renderButtons() only called once

[daynew]

Received a Zendesk ticket where the ImmersiveReaderButton keeps duplicating. This PR adds a flag to make sure the code which adds styling to the ImmersiveReaderButton only happens once.

Links

• Zendesk ticket

Testing story

• Couldn't reproduce the bug :(
• Visited http://localhost-studio.code.org:3000/s/express-2020/stage/2/puzzle/6 to verify the ImmersiveReaderButton still works normally.

Reviewer Checklist:

• Tests provide adequate coverage
• Privacy and Security impacts have been assessed
• Code is well-commented
• New features are translatable or updates will not break translations
• Relevant documentation has been added or updated
• User impact is well-understood and desirable
• Pull Request is labeled appropriately
• Follow-up work items (including potential tech debt) are tracked and linked

[jmkulwik]

I don't have much context, but not seeing any issues :) LGTM