EZP-23748: As an editor, I want to be able to fill the image field

JIRA: https://jira.ez.no/browse/EZP-23748

# Description

This PR adds a field edit view for the Image fields, as a result it's not possible to handle the Image fields content.

Screencast: https://www.youtube.com/watch?v=GuGH5TrmAQU

## Bug / Known limitations

* [No drag and drop yet](https://jira.ez.no/browse/EZP-23789) (to limit the size of this already too big PR)
* Because of [EZP-23788](https://jira.ez.no/browse/EZP-23788), you can not update the alternative text without updating the image
* The image type is not displayed if the image field is already filled because [the REST API does not provide the mime type of the image](https://jira.ez.no/browse/EZP-23758).
* In an ideal world, [the chosen file should uploaded as soon as possible](https://jira.ez.no/browse/EZP-23802) (for instance while the user is filling the rest of the form) but [the REST API is missing a resource for that](https://jira.ez.no/browse/EZP-23801).

## Tasks

* [x] Implement a basic Image field edit view
* [x] Avoid sending the image content even if it was not changed
* [x] Improve the CSS to better handle small images (width < 300px)
* [x] Improve the transitions to make it feel smoother
* [x] Fix the form validation so that the form is updated when the user clicks on publish
* [ ] ~~Drag an drop~~ (will be done later in https://jira.ez.no/browse/EZP-23789)

# Tests

unit tests + manual test
http://caniuse.com/#feat=bloburls tells me it's supported almost everywhere and on that topic, I tend to trust more this site than MDN. But, like for the FileReader comment, we are currently trying to have something working in *most* browsers, we'll fix edge cases and those kind of specific browsers issues later.