Have a live Debian sid desktop ISO image available for download

For debugging purpose, for example to track down hardware issues, it would be helpful to have images of Debian sid available for download to users and testers.

Subtasks

#6350 (closed)
#6351 (closed)
#6353 (closed)
#6352 (closed)

Related issues

Related to #12613 (closed)

Original created by @sajolida on 6349 (Redmine)

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