Add 'variant' field to Part model

- Display 'long_name' which is concatenation of name and variant

InvenTree/part/migrations/0020_auto_20190510_2022.py

@@ -0,0 +1,27 @@
+# Generated by Django 2.2 on 2019-05-10 10:22
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('part', '0019_auto_20190508_2332'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='part',
+            name='variant',
+            field=models.CharField(blank=True, help_text='Part variant or revision code', max_length=32),
+        ),
+        migrations.AlterField(
+            model_name='part',
+            name='name',
+            field=models.CharField(help_text='Part name', max_length=100),
+        ),
+        migrations.AlterUniqueTogether(
+            name='part',
+            unique_together={('name', 'variant')},
+        ),
+    ]