Implemented EZP-20831: image variantions with REST

This pull request implements https://jira.ez.no/browse/EZP-20831, adding support for image variations with the REST API.

The specifications can be found in the issue.

h3. Remaining tasks
- [ ] Refactor REST Content\Controller, extracting this route's method to a ContentImage controller
- [ ] Remove IOService as an injected dependency
- [ ] Add generation of hypermedia links to the image variation resource in vnd.ez.api.Content.