🐛(backend) filter invitation with case insensitive email

A user can be invited and no control is made on the email case. Then,
when a new user is created, we are looking if there are pending
invitation and the lookup used is case sensitive. We change it using
__iexact which is case insensitive.

src/backend/core/models.py

@@ -221,7 +221,7 @@ class User(AbstractBaseUser, BaseModel, auth_models.PermissionsMixin):
         Expired invitations are ignored.
         """
         valid_invitations = Invitation.objects.filter(
-            email=self.email,
+            email__iexact=self.email,
             created_at__gte=(
                 timezone.now()
                 - timedelta(seconds=settings.INVITATION_VALIDITY_DURATION)