Move the unique key on checksums to migration 15

This shouldn't affect anyone, since this migration is pretty old, but it
allows people using PostgreSQL to actually run Paperless.

src/documents/migrations/0014_document_checksum.py

@@ -158,9 +158,4 @@ class Migration(migrations.Migration):
             name='modified',
             field=models.DateTimeField(auto_now=True, db_index=True),
         ),
-        migrations.AlterField(
-            model_name='document',
-            name='checksum',
-            field=models.CharField(editable=False, help_text='The checksum of the original document (before it was encrypted).  We use this to prevent duplicate document imports.', max_length=32, unique=True),
-        ),
     ]

src/documents/migrations/0015_add_insensitive_to_match.py

@@ -12,6 +12,11 @@ class Migration(migrations.Migration):
     ]
 
     operations = [
+        migrations.AlterField(
+            model_name='document',
+            name='checksum',
+            field=models.CharField(editable=False, help_text='The checksum of the original document (before it was encrypted).  We use this to prevent duplicate document imports.', max_length=32, unique=True),
+        ),
         migrations.AddField(
             model_name='correspondent',
             name='is_insensitive',