Issue #1996714 by amateescu: Convert FileItem and ImageItem to extend EntityReferenceItem.

...ent_translation/Tests/ContentTranslationSyncImageTest.php

...d/Plugin/Type/FieldType/ConfigEntityReferenceItemBase.php

core/modules/file/file.api.php

core/modules/file/file.field.inc

core/modules/file/file.install

core/modules/file/file.views.inc

.../Drupal/file/Plugin/field/formatter/FileFormatterBase.php

...s/file/lib/Drupal/file/Plugin/field/widget/FileWidget.php

...dules/file/lib/Drupal/file/Tests/FileFieldDisplayTest.php

.../modules/file/lib/Drupal/file/Tests/FileFieldPathTest.php

...es/file/lib/Drupal/file/Tests/FileFieldRSSContentTest.php

...ules/file/lib/Drupal/file/Tests/FileFieldRevisionTest.php

...ules/file/lib/Drupal/file/Tests/FileFieldValidateTest.php

...odules/file/lib/Drupal/file/Tests/FileFieldWidgetTest.php

core/modules/file/lib/Drupal/file/Tests/FileItemTest.php

core/modules/file/lib/Drupal/file/Tests/FilePrivateTest.php

...dules/file/lib/Drupal/file/Tests/FileTokenReplaceTest.php

core/modules/file/lib/Drupal/file/Type/FileItem.php

core/modules/image/image.field.inc

core/modules/image/image.install