Fix EZP-20769: hardcoded path to image variants in REST

See https://jira.ez.no/browse/EZP-20769.

Path to variants was included in REST output with an Accept header vnd.api.ez.Content, but weren't properly generated. On top of that, it was referencing the path to images that may actually not exist.

Further specifications & improvements are required on that part. Follow-up story: https://jira.ez.no/browse/EZP-20831.