Reintroduce WhisperBack in Jessie ISOs

WhisperBack is not installable on Jessie, so it was removed (commits f7a46c27b and 5ae5b11). Once it’s installable again, we’ll want to revert these commits.

Feature Branch: feature/7756-reintroduce-whisperback

Subtasks

#8723 (closed)
#8724 (closed)

Related issues

Blocked by #7755 (closed)

Original created by @intrigeri on 7756 (Redmine)

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