Move __onload-open modifier to the parent component class

[anselmbradford]

Doesn't make sense to put a modifier inside the component, it should be a modifier overall on o-expandable. This moves the modifier to the component from content.

Changes

• Move __onload-open modifier to the parent component class

Testing

1. PR preview expandable should work as before.

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[netlify]

✅ Thanks for the improvements! Browse a preview of your changes using the link below.

Name	Link
🔨 Latest commit	a544a06
🔍 Latest deploy log	https://app.netlify.com/sites/cfpb-design-system/deploys/643f00e296302c000883406f
😎 Deploy Preview	https://deploy-preview-1632--cfpb-design-system.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

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