Add 'revision' field to part

- e.g. different versions of a product
- Can keep old versions in database and mark as inactive

InvenTree/part/migrations/0011_part_revision.py

@@ -0,0 +1,18 @@
+# Generated by Django 2.2.2 on 2019-06-20 11:37
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('part', '0010_auto_20190620_2135'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='part',
+            name='revision',
+            field=models.CharField(blank=True, help_text='Part revision or version number', max_length=100),
+        ),
+    ]