Download all sponsors logo as one zip

[HugoGresse]

• add new button to download a sponsor category images as one zip
• add confirm dialog when deleting a sponsor category

[github-actions[bot]]

Visit the preview URL for this PR (updated for commit e39f940):

• https://conferencecenterr--pr126-download-button-imag-xen67iz6.web.app
• https://apiopenplanner--pr126-download-button-imag-dwo8br9r.web.app

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