Implemented EZP-20831: image variations with REST

This pull request implements https://jira.ez.no/browse/EZP-20831, adding support for image variations with the REST API.

### Specifications
https://github.com/bdunogier/ezpublish-kernel/blob/fix-EZP-20831-rest_image_variants/doc/specifications/rest/REST-API-V2.rst#13256load-an-image-variation

### Remaining tasks
- [x] Refactor REST Content\Controller, extracting this route's method to a ContentImage controller
- [x] Remove IOService as an injected dependency
- [x] Add generation of hypermedia links to the image variation resource in vnd.ez.api.Content.
- [x] Fix unit tests affected by the addition of imageId to Image\Value
- [x] Fix generation of self-href in ImageVariation
- [x] Rename variant to variation