Initial updates required to run under django 2.2

- Change to middleware.py
- Change some URLs formatting
- Fix django version at 2.2

InvenTree/stock/migrations/0004_auto_20190412_2030.py

@@ -0,0 +1,19 @@
+# Generated by Django 2.2 on 2019-04-12 10:30
+
+import django.core.validators
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('stock', '0003_auto_20180510_1042'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='stockitem',
+            name='status',
+            field=models.PositiveIntegerField(choices=[(10, 'OK'), (50, 'Attention needed'), (55, 'Damaged'), (60, 'Destroyed')], default=10, validators=[django.core.validators.MinValueValidator(0)]),
+        ),
+    ]