🔧(project) add DJANGO_EMAIL_URL_APP environment variable

Most of Docs app is configured thanks to environment
variables, except the url in the email that
was from the django site table.
Now we can set it with DJANGO_EMAIL_URL_APP
environment variable to have a better consistency.
We keep the previous way to avoid breaking
changes.

src/backend/core/models.py

@@ -817,7 +817,7 @@ class Document(MP_Node, BaseModel):
     def send_email(self, subject, emails, context=None, language=None):
         """Generate and send email from a template."""
         context = context or {}
-        domain = Site.objects.get_current().domain
+        domain = settings.EMAIL_URL_APP or Site.objects.get_current().domain
         language = language or get_language()
         context.update(
             {