♻️(backend) refacto email invitation

Remove email invitation from Invitation model
to be able to use it in other context.
We add it in utils.py instead, and it will be called
from the viewset.
We add the document_id to link to the document from
the mail.

src/backend/core/api/viewsets.py

@@ -1,4 +1,5 @@
 """API endpoints"""
+from core.utils import email_invitation
 from django.contrib.postgres.aggregates import ArrayAgg
 from django.db.models import (
     OuterRef,
@@ -598,3 +599,10 @@ class InvitationViewset(
                 .distinct()
             )
         return queryset
+    
+    def perform_create(self, serializer):
+        """Save invitation to a document then send an email to the invited user."""
+        invitation = serializer.save()
+
+        language = self.request.headers.get("Content-Language", "en-us")
+        email_invitation(language, invitation.email, invitation.document.id)