user contan screening_number_prefix for adding subjects

smash/web/models/worker.py

@@ -36,6 +36,12 @@ class Worker(models.Model):
     email = models.EmailField(
         verbose_name='E-mail'
     )
+    screening_number_prefix = models.CharField(max_length=1,
+        null=True,
+        blank=True,
+        verbose_name='Default screening number prefix',
+    )
+
     ROLE_CHOICES_SECRETARY = "SECRETARY"
     ROLE_CHOICES = (
         ('DOCTOR', 'Doctor'),