refs https://github.com/TryGhost/Product/issues/3831
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### <samp>🤖 Generated by Copilot at f85ceff</samp>
This pull request adds drag and drop functionality and custom styling
options to the `FileUpload` and `ImageUpload` components, and uses them
to improve the user experience of uploading themes and user images in
the admin settings. It also fixes a minor issue with the background
color contrast in the `IntegrationHeader` component in dark mode, and
updates the corresponding tests.