Translate the toast message when a user is created or invited

Pre-requisite : change the language of the browser to French (or another language than English)

Steps to reproduce:

1. As an admin, go to Administration > Users & groups (I keep the english label to ease the scenario)
2. Hit the New button then Users
3. Fill all the fields and submit the form
4. Observe that a toast is displayed in the bottom left corner with the message "User xxx created"

Expected behavior: the message in the toast should be "Utilisateur xxx créé"

This is due to the fact that the message is not translated around these lines: https://github.com/nuxeo/nuxeo-elements/blob/maintenance-3.0.x/ui/nuxeo-user-group-management.js#L304-L313

The same bug affects the groups. But a fix for users should also fix the case of the groups