make name unique for Tags

gracedb/events/migrations/0020_tag_unique_name.py

+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.5 on 2018-04-15 21:05
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('events', '0019_event_superevent'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='tag',
+            name='name',
+            field=models.CharField(max_length=100, unique=True),
+        ),
+    ]

gracedb/events/models.py

@@ -807,7 +807,8 @@ class Tag(CleanSaveModel):
     assumption that this will always be done.  But is it really important to
     track those things?  Doesn't seem like it.
     """
-    name = models.CharField(max_length=100, null=False, blank=False)
+    name = models.CharField(max_length=100, null=False, blank=False,
+        unique=True)
     displayName = models.CharField(max_length=200, null=True, blank=True)
 
     def __unicode__(self):