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/migrations/0078_auto_20171206_1603.py

@@ -0,0 +1,59 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.2 on 2017-12-06 16:03
+from __future__ import unicode_literals
+
+import django.db.models.deletion
+from django.conf import settings
+from django.db import migrations, models
+
+import pretix.base.models.auth
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pretixbase', '0077_auto_20171124_1629'),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='NotificationSetting',
+            fields=[
+                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('action_type', models.CharField(max_length=255)),
+                ('method', models.CharField(choices=[('mail', 'E-mail')], max_length=255)),
+                ('event', models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.CASCADE, to='pretixbase.Event')),
+                ('user', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, to=settings.AUTH_USER_MODEL)),
+                ('enabled', models.BooleanField(default=True)),
+            ],
+        ),
+        migrations.AlterUniqueTogether(
+            name='notificationsetting',
+            unique_together=set([('user', 'action_type', 'event', 'method')]),
+        ),
+        migrations.AddField(
+            model_name='logentry',
+            name='visible',
+            field=models.BooleanField(default=True),
+        ),
+        migrations.AlterField(
+            model_name='notificationsetting',
+            name='event',
+            field=models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.CASCADE, related_name='notification_settings', to='pretixbase.Event'),
+        ),
+        migrations.AlterField(
+            model_name='notificationsetting',
+            name='user',
+            field=models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='notification_settings', to=settings.AUTH_USER_MODEL),
+        ),
+        migrations.AddField(
+            model_name='user',
+            name='notifications_send',
+            field=models.BooleanField(default=True, help_text='If turned off, you will not get any notifications.', verbose_name='Receive notifications according to my settings below'),
+        ),
+        migrations.AddField(
+            model_name='user',
+            name='notifications_token',
+            field=models.CharField(default=pretix.base.models.auth.generate_notifications_token, max_length=255),
+        ),
+    ]