DEV: process image uploads in the Zendesk API import script (PR #14524)

When migrating from the Zendesk API, images look like this in a post’s raw body: ![](https://<sub-domain>.zendesk.com/<hash>.<image-format>)

Use the Zendesk URL to download the image and create the upload in Discourse.

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Looks good to me. :+1:t3:

Make sure to sign the CLA and fix the lint error.