Fix the images in the theme builder to use permanent URI

[abidlabs]

The header image in the theme builder was referencing a file from our GitHub repo which was moved due to recent refactoring. I've uploaded the image to the S3 bucket and updated the references.

[gradio-pr-bot]

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Install Gradio from this PR

pip install https://gradio-builds.s3.amazonaws.com/4ce78fe5c21c985a866977cac05f1f729c75610a/gradio-3.40.1-py3-none-any.whl

Install Gradio Python Client from this PR

pip install "gradio-client @ git+https://github.com/gradio-app/gradio@4ce78fe5c21c985a866977cac05f1f729c75610a#subdirectory=client/python"

[gradio-pr-bot]

🦄 change detected

This Pull Request includes changes to the following packages.

Package	Version
gradio	patch

• Maintainers can select this checkbox to manually select packages to update.

With the following changelog entry.

Fix the images in the theme builder to use permanent URI

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[freddyaboulton]

Thanks @abidlabs !

[abidlabs]

Thanks @freddyaboulton!