[11.9 EE port] Make asset image computation simpler

Balasankar 'Balu' Crequested to merge
asset-image-slug-11-9-ee into 11-9-stable-ee

11.9 port of !3110 (merged). Depends on https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10843 going in first.

Edited by Balasankar 'Balu' C

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