Implement notifications for admin users (#700)

* First stab at notification settings

* Add "global" setting for notification levels

* Trigger notification task

* Get users with permission for event

* Actually send notification emails

* More notifications

* Allow to turn off notifications

* Link in email to pause all notifications

* Add NotificationType to wordlist

* Add notification tests

* Add documentation

* Rebase fixes

src/pretix/base/models/notifications.py

@@ -0,0 +1,36 @@
+from django.db import models
+from django.utils.translation import ugettext_lazy as _
+
+
+class NotificationSetting(models.Model):
+    """
+    Stores that a user wants to get notifications of a certain type via a certain
+    method for a certain event. If event is None, the notification shall be sent
+    for all events the user has access to.
+
+    :param user: The user to nofify.
+    :type user: User
+    :param action_type: The type of action to notify for.
+    :type action_type: str
+    :param event: The event to notify for.
+    :type event: Event
+    :param method: The method to notify with.
+    :type method: str
+    :param enabled: Indicates whether the specified notification is enabled. If no
+                    event is set, this must always be true. If no event is set, setting
+                    this to false is equivalent to deleting the object.
+    :type enabled: bool
+    """
+    CHANNELS = (
+        ('mail', _('E-mail')),
+    )
+    user = models.ForeignKey('User', on_delete=models.CASCADE,
+                             related_name='notification_settings')
+    action_type = models.CharField(max_length=255)
+    event = models.ForeignKey('Event', null=True, blank=True, on_delete=models.CASCADE,
+                              related_name='notification_settings')
+    method = models.CharField(max_length=255, choices=CHANNELS)
+    enabled = models.BooleanField(default=True)
+
+    class Meta:
+        unique_together = ('user', 'action_type', 'event', 'method')