♻️(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/utils.py

@@ -0,0 +1,39 @@
+"""
+Utilities for the core app.
+"""
+import smtplib
+from logging import getLogger
+
+from django.conf import settings
+from django.contrib.sites.models import Site
+from django.core.mail import send_mail
+from django.template.loader import render_to_string
+from django.utils.translation import gettext_lazy as _
+from django.utils.translation import override
+
+logger = getLogger(__name__)
+
+
+def email_invitation(language, email, document_id):
+    """Send email invitation."""
+    try:
+        with override(language):
+            title = _("Invitation to join Docs!")
+            template_vars = {
+                "title": title,
+                "site": Site.objects.get_current(),
+                "document_id": document_id,
+            }
+            msg_html = render_to_string("mail/html/invitation.html", template_vars)
+            msg_plain = render_to_string("mail/text/invitation.txt", template_vars)
+            send_mail(
+                title,
+                msg_plain,
+                settings.EMAIL_FROM,
+                [email],
+                html_message=msg_html,
+                fail_silently=False,
+            )
+
+    except smtplib.SMTPException as exception:
+        logger.error("invitation to %s was not sent: %s", email, exception)