Migrate from constance to dynamic-preferences

Rationale: django-dynamic-preferences allows defining global preferences and "instance preferences", i.e. user preferences, the same way. It is also way more sophisticated than constance and provides more possibilities for preference fields as well as rendering options.

Closes #162 (closed), advances #214 (closed) and others.

aleksis/core/util/notifications.py

@@ -38,7 +38,7 @@ def _send_notification_email(notification: "Notification", template: str = "noti
     }
     send_templated_mail(
         template_name=template,
-        from_email=lazy_preference("notification__mail_out"),
+        from_email=lazy_preference("mail", "address"),
         recipient_list=[notification.recipient.email],
         context=context,
     )
@@ -63,7 +63,7 @@ def _send_notification_sms(
 # - Check for availability
 # - Send notification through it
 _CHANNELS_MAP = {
-    "email": (_("E-Mail"), lambda: lazy_preference("notification__mail_out"), _send_notification_email),
+    "email": (_("E-Mail"), lambda: lazy_preference("mail", "address"), _send_notification_email),
     "sms": (_("SMS"), lambda: getattr(settings, "TWILIO_SID", None), _send_notification_sms),
 }
 
@@ -75,7 +75,7 @@ def send_notification(notification: Union[int, "Notification"], resend: bool = F
     previously marked as sent.
     """
 
-    channels = lazy_preference("notification__channels")
+    channels = lazy_preference("notification", "channels")
 
     if isinstance(notification, int):
         Notification = apps.get_model("core", "Notification")