i18n: element form, rename the key to group them in one place

src/Controller/ElementFormController.php

src/Controller/UserController.php

templates/element-form/element-form.html.twig

templates/element-form/form-partials/address.html.twig

templates/element-form/form-partials/admin.html.twig

templates/element-form/form-partials/open-hours.html.twig

...es/element-form/form-partials/validate-and-send.html.twig

templates/header.html.twig

templates/home.html.twig

translations/messages+intl-icu.fr.yaml