refactor flash messaging, added item name to flash messages
removed setFlash method
renamed variables to follow coding standards
Merge remote-tracking branch 'origin/master' into item_name_in_flashes

Conflicts:
	Controller/CRUDController.php
	Resources/views/standard_layout.html.twig
cs
Trigger event before removing element
Support for class-level validation

This allows for

    $errorElement->addConstraint(new UniqueEntity(['fields' => 'email']));

and similar code without an enclosing ->with().
Update ModelToIdTransformer.php

This is to fix problems with entities with 0 as id, empty will return true if u give 0 as parameter
adding unit test for ModelToIdTransformer in the cases where it gets 0 as id
removed test class from the file
Implemented getAdminsByGroup() / hasGroup() functions
Test if identifier is null or not in twig templates, to determine if we have to generate an edit/create form

Fix Issue #1466 create/update routing problem in form creation, when id is 0
Update preview_mode.rst
Update preview_mode.rst
added some preview mode screenshots
cropped screens
Update preview_mode.rst
Update preview_mode.rst
Update preview_mode.rst
empty_value translation