Allow to send e-mails to attendees individually (#1299)

* .

* Add a position detail page to the frontend

* Mail templates

* Send mails

* Send reminder email

* Add position support to sendmail plugin

* Add and fix some tests

* Fix failing test on real databases

src/pretix/base/migrations/0122_orderposition_web_secret.py

@@ -0,0 +1,20 @@
+# Generated by Django 2.2.1 on 2019-05-15 13:23
+
+from django.db import migrations, models
+
+import pretix.base.models.orders
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pretixbase', '0121_order_email_known_to_work'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='orderposition',
+            name='web_secret',
+            field=models.CharField(db_index=True, default=pretix.base.models.orders.generate_secret, max_length=32),
+        ),
+    ]