relax membership to allow unresolved people

relates to opencivicdata/pupa#253

opencivicdata/migrations/0006_auto_20170220_2057.py

@@ -0,0 +1,25 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.10.4 on 2017-02-20 20:57
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('opencivicdata', '0005_add_agenda_item_classification'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='membership',
+            name='person_name',
+            field=models.CharField(blank=True, default='', max_length=300),
+        ),
+        migrations.AlterField(
+            model_name='membership',
+            name='person',
+            field=models.ForeignKey(null=True, on_delete=models.deletion.CASCADE, related_name='memberships', to='opencivicdata.Person'),
+        ),
+    ]

opencivicdata/models/people_orgs.py

@@ -203,7 +203,8 @@ class PersonSource(LinkBase):
 class Membership(OCDBase):
     id = OCDIDField(ocd_type='membership')
     organization = models.ForeignKey(Organization, related_name='memberships')
-    person = models.ForeignKey(Person, related_name='memberships')
+    person = models.ForeignKey(Person, related_name='memberships', null=True)
+    person_name = models.CharField(max_length=300, blank=True, default='')
     post = models.ForeignKey(Post, related_name='memberships', null=True)
     on_behalf_of = models.ForeignKey(Organization, related_name='memberships_on_behalf_of',
                                      null=True)