Generalise notifications and implement SMS notifications

This MR implements some generalisations for the notification system.

1. Use more generic naming in model
2. Create utility library ofr sending notifications
3. Implement SMS channel (Twilio) in addition to e-mail channel
4. Make channels configurable in admin backend

Special attention has been paid to:

• Allow easy addition of channels
• Ensure only configured channels can be selected
• Ensure channels that have been selected, but have becom unavailable, are not tried

>>> from aleksis.core.models import Person, Notification
>>> nik = Person.objects.first()
>>> noti = Notification.objects.create(title="Tag des Raclettes", description="Bitte überbacken Sie Ihre Vertretungsaufgaben umgehend mit Käse 🧀!", link="http://www.raclette.de/shop/", recipient=nik, sender="Feiertage-App")

Closes #165 (closed)

aleksis/core/util/notifications.py

@@ -9,6 +9,7 @@ from django.utils.functional import lazy
 from django.utils.translation import gettext_lazy as _
 
 from templated_email import send_templated_mail
+
 try:
     from twilio.rest import Client as TwilioClient
 except ImportError:
@@ -17,7 +18,9 @@ except ImportError:
 from .core_helpers import celery_optional, lazy_config
 
 
-def send_templated_sms(template_name: str, from_number: str, recipient_list: Sequence[str], context: dict):
+def send_templated_sms(
+    template_name: str, from_number: str, recipient_list: Sequence[str], context: dict
+):
     """ Render a plan-text template and send via SMS to all recipients. """
 
     template = get_template(template_name)
@@ -41,7 +44,9 @@ def _send_notification_email(notification: "Notification", template: str = "noti
     )
 
 
-def _send_notification_sms(notification: "Notification", template: str = "sms/notification.txt") -> None:
+def _send_notification_sms(
+    notification: "Notification", template: str = "sms/notification.txt"
+) -> None:
     context = {
         "notification": notification,
         "notification_user": notification.recipient.adressing_name,
@@ -98,4 +103,5 @@ def get_notification_choices() -> list:
             choices.append((channel, name))
     return choices
 
+
 get_notification_choices_lazy = lazy(get_notification_choices, tuple)