BinaryFileEditView and ImageEditView refactoring

Follow-up of https://jira.ez.no/browse/EZP-23749

# Description

This patch tries to re-factor the BinaryFile and Image field edit views to share more code and to ease the future implementation of the Media field edit view and the drag and drop feature on those three fields.

## Tasks

* [x] Re-factor the JS code to have a common base class
* [x] Avoid CSS duplication
* [x] Re-factor/re-organize the unit tests

# Tests

manual tests + unit tests
aaah that's perfectly right!