python: Sort migrations/management command imports with isort.

This is a preparatory commit for using isort for sorting all of our
imports, merging changes to files where we can easily review the
changes as something we're happy with.

These are also files with relatively little active development, which
means we don't expect much merge conflict risk from these changes.

analytics/migrations/0012_add_on_delete.py

@@ -2,8 +2,8 @@
 # Generated by Django 1.11.6 on 2018-01-29 08:14
 from __future__ import unicode_literals
 
-from django.db import migrations, models
 import django.db.models.deletion
+from django.db import migrations, models
 
 
 class Migration(migrations.Migration):