attachment: Make path_id field unique.

This will help avoid future bugs similar to that fixed in migration
0072.

zerver/migrations/0075_attachment_path_id_unique.py

@@ -0,0 +1,25 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.10.5 on 2017-04-13 22:29
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('zerver', '0074_fix_duplicate_attachments'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='archivedattachment',
+            name='path_id',
+            field=models.TextField(db_index=True, unique=True),
+        ),
+        migrations.AlterField(
+            model_name='attachment',
+            name='path_id',
+            field=models.TextField(db_index=True, unique=True),
+        ),
+    ]

zerver/models.py

@@ -1226,7 +1226,7 @@ class AbstractAttachment(ModelReprMixin, models.Model):
     # path_id is a storage location agnostic representation of the path of the file.
     # If the path of a file is http://localhost:9991/user_uploads/a/b/abc/temp_file.py
     # then its path_id will be a/b/abc/temp_file.py.
-    path_id = models.TextField(db_index=True)  # type: Text
+    path_id = models.TextField(db_index=True, unique=True)  # type: Text
     owner = models.ForeignKey(UserProfile)  # type: UserProfile
     realm = models.ForeignKey(Realm, blank=True, null=True)  # type: Realm
     is_realm_public = models.BooleanField(default=False)  # type: bool