registration: add support for year of birth

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

dav_registration/models.py

@@ -40,6 +40,12 @@ class Registration(models.Model):
     phone_number = models.CharField(max_length=254,
                                     verbose_name=_('Telefonnummer'),
                                     help_text=_('Idealerweise eine Mobilfunk-Nummer'))
+
+    year_of_birth = models.IntegerField(verbose_name=_('Geburtsjahr'),
+                                        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).'))
+
     dav_member = models.BooleanField(default=True,
                                      verbose_name=_('DAV Mitglied'))
     dav_number = models.CharField(max_length=62,
@@ -74,6 +80,11 @@ class Registration(models.Model):
 
     answered_obsolete = models.BooleanField(default=False, verbose_name=_('Durch Tourleitung beantwortet'))
 
+    def approx_age(self):
+        now = datetime.datetime.now()
+        year_now = now.year
+        return year_now - self.year_of_birth
+
     @staticmethod
     def pk2hexstr(pk):
         return hex(pk * 113)[2:]  # 113 has no meaning, but it produce nice looking hex codes.
@@ -113,6 +124,7 @@ class Registration(models.Model):
 {address}, {postal_code} {city}
 
 DAV Mitglied: {dav_info}
+Jahrgang: {year_of_birth} (ungefähres Alter: {approx_age})
 
 Erfahrung:
 {experience}
@@ -131,6 +143,8 @@ Anmerkung:
             postal_code=self.postal_code,
             city=self.city,
             dav_info=dav_info,
+            year_of_birth=self.year_of_birth,
+            approx_age=self.approx_age(),
             experience=self.experience,
             note=self.note,
         )