[import] Automatically proxy the images

[kptdobe]

The Importer Guidelines describe how image can be proxied if they cannot be uploaded.
This logic could be added by default, .i.e. for all images, remove the host from the src tag and append a ?host= query param. Like this, images would be proxied by the localhost proxy server.

This would work as is but images from different hosts would be stored in the same folder structure. We probably need adobe/helix-cli#2072 for a clean approach.