Add unique constraint to Staff.username.

Previously had some duplicates for "pono" which were causing
MultipleObjectsReturned errors. A username is generally considered to be unique.

www/conservancy/apps/staff/migrations/0001_initial.py

@@ -0,0 +1,31 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.10.7 on 2021-11-28 21:12
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    initial = True
+
+    dependencies = [
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='Person',
+            fields=[
+                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('username', models.CharField(max_length=20)),
+                ('formal_name', models.CharField(max_length=200)),
+                ('casual_name', models.CharField(max_length=200)),
+                ('currently_employed', models.BooleanField(default=True)),
+                ('date_created', models.DateTimeField(auto_now_add=True)),
+                ('date_last_modified', models.DateTimeField(auto_now=True)),
+            ],
+            options={
+                'verbose_name_plural': 'people',
+            },
+        ),
+    ]

www/conservancy/apps/staff/migrations/0002_auto_20211128_2112.py

@@ -0,0 +1,20 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.10.7 on 2021-11-28 21:12
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('staff', '0001_initial'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='person',
+            name='username',
+            field=models.CharField(max_length=20, unique=True),
+        ),
+    ]

www/conservancy/apps/staff/models.py

@@ -7,7 +7,7 @@ class Person(models.Model):
     Referenced from other models (blog, events, etc)
     """
 
-    username = models.CharField(max_length=20)
+    username = models.CharField(max_length=20, unique=True)
     formal_name = models.CharField(max_length=200)
     casual_name = models.CharField(max_length=200)
 #    title = models.CharField(max_length=200, blank=True)
@@ -28,4 +28,3 @@ class Person(models.Model):
 
     def biography_url(self):
         return u"/about/#%s" % self.username
-