✨(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/models.py

@@ -4,22 +4,28 @@ Declare and configure the models for the People core application
 
 import json
 import os
+import smtplib
 import uuid
 from datetime import timedelta
+from logging import getLogger
 
 from django.conf import settings
 from django.contrib.auth import models as auth_models
 from django.contrib.auth.base_user import AbstractBaseUser
 from django.core import exceptions, mail, validators
 from django.db import models
+from django.template.loader import render_to_string
 from django.utils import timezone
 from django.utils.functional import lazy
 from django.utils.text import slugify
 from django.utils.translation import gettext_lazy as _
+from django.utils.translation import override
 
 import jsonschema
 from timezone_field import TimeZoneField
 
+logger = getLogger(__name__)
+
 current_dir = os.path.dirname(os.path.abspath(__file__))
 contact_schema_path = os.path.join(current_dir, "jsonschema", "contact_data.json")
 with open(contact_schema_path, "r", encoding="utf-8") as contact_schema_file:
@@ -516,6 +522,8 @@ class Invitation(BaseModel):
             raise exceptions.PermissionDenied()
         super().save(*args, **kwargs)
 
+        self.email_invitation()
+
     def clean(self):
         """Validate fields."""
         super().clean()
@@ -559,3 +567,24 @@ class Invitation(BaseModel):
             "patch": False,
             "put": False,
         }
+
+    def email_invitation(self):
+        """Email invitation to the user."""
+        try:
+            with override(self.issuer.language):
+                template_vars = {
+                    "title": _("Invitation to join Desk!"),
+                }
+                msg_html = render_to_string("mail/html/invitation.html", template_vars)
+                msg_plain = render_to_string("mail/text/invitation.txt", template_vars)
+                mail.send_mail(
+                    _("Invitation to join Desk!"),
+                    msg_plain,
+                    settings.EMAIL_FROM,
+                    [self.email],
+                    html_message=msg_html,
+                    fail_silently=False,
+                )
+
+        except smtplib.SMTPException as exception:
+            logger.error("invitation to %s was not sent: %s", self.email, exception)