Send an email when a workflow is send to an external

- create an event subscriber to catch the workflow which arrive to a "sentExternal" step;
- add a messenger's message to handle the generation of the email;
- add a simple message, and a simple controller for viewing the document
- add dedicated tests

src/Bundle/ChillMainBundle/translations/messages+intl-icu.fr.yaml

@@ -67,6 +67,8 @@ workflow:
             one {Signature demandée}
             other {Signatures demandées}
         }
+    send_external_message:
+        document_available_until: Le lien sera valable jusqu'au {expiration, date, long} à {expiration, time, short}.
 
 duration:
     minute: >-