Add event selection endpoint (#1827)

* Add event selection endpoint

* Minor fixes

* Add filter by gate

src/pretix/base/migrations/0169_checkinlist_gates.py

@@ -0,0 +1,18 @@
+# Generated by Django 3.0.10 on 2020-10-24 15:55
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pretixbase', '0168_auto_20201023_1447'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='checkinlist',
+            name='gates',
+            field=models.ManyToManyField(to='pretixbase.Gate'),
+        ),
+    ]