✨(backend) email invitation to new users

When generating an Invitation object within the database, our intention
is to promptly notify the user via email. We send them an invitation
to join Desk.

This code is inspired by Joanie successful order flow.

Johann's design was missing a link to Desk, I simply added a button which
redirect to the staging url. This url is hardcoded, we should refactor it
when we will deploy Desk in pre-prod or prod environments.

Johann's design relied on Marianne font. I implemented a simpler version,
which uses a google font. That's not important for MVP.

Look and feel of this first invitation template is enough to make our PoC
functionnal, which is the more important.

src/backend/core/tests/test_models_invitations.py

@@ -2,11 +2,14 @@
 Unit tests for the Invitation model
 """
 
+import smtplib
 import time
 import uuid
+from logging import Logger
+from unittest import mock
 
 from django.contrib.auth.models import AnonymousUser
-from django.core import exceptions
+from django.core import exceptions, mail
 
 import pytest
 from faker import Faker
@@ -236,3 +239,64 @@ def test_models_team_invitations_get_abilities_member():
         "patch": False,
         "put": False,
     }
+
+
+def test_models_team_invitations_email():
+    """Check email invitation during invitation creation."""
+
+    member_access = factories.TeamAccessFactory(role="member")
+    team = member_access.team
+
+    # pylint: disable-next=no-member
+    assert len(mail.outbox) == 0
+
+    factories.TeamAccessFactory(team=team)
+    invitation = factories.InvitationFactory(team=team, email="john@people.com")
+
+    # pylint: disable-next=no-member
+    assert len(mail.outbox) == 1
+
+    # pylint: disable-next=no-member
+    (email,) = mail.outbox
+
+    assert email.to == [invitation.email]
+    assert email.subject == "Invitation to join Desk!"
+
+    email_content = " ".join(email.body.split())
+    assert "Invitation to join Desk!" in email_content
+
+
+@mock.patch(
+    "django.core.mail.send_mail",
+    side_effect=smtplib.SMTPException("Error SMTPException"),
+)
+@mock.patch.object(Logger, "error")
+def test_models_team_invitations_email_failed(mock_logger, _mock_send_mail):
+    """Check invitation behavior when an SMTP error occurs during invitation creation."""
+
+    member_access = factories.TeamAccessFactory(role="member")
+    team = member_access.team
+
+    # pylint: disable-next=no-member
+    assert len(mail.outbox) == 0
+
+    factories.TeamAccessFactory(team=team)
+
+    # No error should be raised
+    invitation = factories.InvitationFactory(team=team, email="john@people.com")
+
+    # No email has been sent
+    # pylint: disable-next=no-member
+    assert len(mail.outbox) == 0
+
+    # Logger should be called
+    mock_logger.assert_called_once()
+
+    (
+        _,
+        email,
+        exception,
+    ) = mock_logger.call_args.args
+
+    assert email == invitation.email
+    assert isinstance(exception, smtplib.SMTPException)