Configuration options for default Rekognition behaviour

[roborourke]

Rekognition by default will attempt to auto populate image alt text among other things. We should provide configuration options to disable this feature or perhaps prevent it on sites with a default language other than english.

https://github.com/humanmade/aws-rekognition/issues/17

Hopefully we can integrate AWS Translate in the near future.

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Acceptance criteria:

• [x] Rekognition features can be configured in composer.json
  • labels, default on
  • moderation labels, default off
  • face detection, default off
  • celebrity recognition, default off
  • text extraction, default off
  • automatic alt text population- if false return an empty string from this filter on an early priority)
• [x] Documentation of configuration options and features
  • Include how to access the metadata for each feature (I think we should add helper functions for getting these in the upstream Rekognition plugin ideally)

[roborourke]

All of this except for the default alt text was done in #20 - will add a separate issue for that.