URLs in mails were missing domains

Signed-off-by: Yohann D'ANELLO <yohann.danello@animath.fr>

apps/participation/views.py

@@ -1,11 +1,12 @@
 from io import BytesIO
 from zipfile import ZipFile
 
+from django.contrib.sites.models import Site
+
 from corres2math.lists import get_sympa_client
 from corres2math.matrix import Matrix
 from corres2math.views import AdminMixin
 from django.contrib.auth.mixins import LoginRequiredMixin
-from django.contrib.sites.models import Site
 from django.core.exceptions import PermissionDenied
 from django.core.mail import send_mail
 from django.db import transaction