Add support for SO numbers

Adds the possibility of using SO numbers when registering in GREG.

greg/migrations/0024_alter_person_gender.py

+# Generated by Django 4.1.2 on 2022-10-13 13:26
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("greg", "0023_identity_invalid"),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name="person",
+            name="gender",
+            field=models.CharField(
+                blank=True,
+                choices=[("male", "Male"), ("female", "Female")],
+                max_length=6,
+                null=True,
+            ),
+        ),
+    ]