exportAllNotes should use POST method for security

👋 Hello, we've received a report for a potential high severity security issue in your repository.

#### Next Steps

1️⃣ Visit **https://huntr.dev/bounties/1-other-hackmdio/codimd** for more advisory information.

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3️⃣ Propose a patch or outsource it to our community.

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*This issue was automatically generated by [huntr.dev](https://huntr.dev) - a bug bounty board for securing open source code.*