EZP-23718: As an editor, I want to see the content of the Image fields in the PlatformUI

JIRA: https://jira.ez.no/browse/EZP-23718

# Description

The goal to add the view to display the Image fields in PlatformUI.

Screencast: http://youtu.be/WD_osx9CShg

## Tasks

* [x] Add a view extension to bring the concept of asynchronously rendered view
* [x] Refactor the Relation and RelationList views with this extension
* [x] Implement the missing API in the JS REST client to deal with image variations ([EZP-22017](https://jira.ez.no/browse/EZP-22017))
* [x] Implement the Image field view
* [x] Add a custom image alias in PlatformUI and use it

# Tests

unit tests + manual tests