registration: add support for year of birth

this is only in dav_registration for now. dav_events is following.

dav_registration/migrations/0009_registration_year_of_birth.py

@@ -0,0 +1,21 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.29 on 2020-12-09 09:20
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('dav_registration', '0008_auto_20201208_1906'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='registration',
+            name='year_of_birth',
+            field=models.IntegerField(default=1870, help_text='Manchmal müssen wir wissen, wie alt unsere Teilnehmer sind. Darum brauchen wir die vierstellige Jahreszahl, des Jahres in dem du geboren bist (zb. 1991).', verbose_name='Geburtsjahr'),
+            preserve_default=False,
+        ),
+    ]