Simplify part model

- Remove 'varian't or ''revision' field
- Part name must be unique across database

InvenTree/part/migrations/0005_auto_20190526_1119.py

@@ -0,0 +1,26 @@
+# Generated by Django 2.2 on 2019-05-26 01:19
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('part', '0004_auto_20190525_2356'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='part',
+            name='revision',
+            field=models.CharField(blank=True, help_text='Part rerevision code', max_length=32),
+        ),
+        migrations.AlterUniqueTogether(
+            name='part',
+            unique_together={('name', 'revision')},
+        ),
+        migrations.RemoveField(
+            model_name='part',
+            name='variant',
+        ),
+    ]

InvenTree/part/migrations/0006_auto_20190526_1215.py

@@ -0,0 +1,27 @@
+# Generated by Django 2.2 on 2019-05-26 02:15
+
+import InvenTree.validators
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('part', '0005_auto_20190526_1119'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='part',
+            name='name',
+            field=models.CharField(help_text='Part name (must be unique)', max_length=100, unique=True, validators=[InvenTree.validators.validate_part_name]),
+        ),
+        migrations.AlterUniqueTogether(
+            name='part',
+            unique_together=set(),
+        ),
+        migrations.RemoveField(
+            model_name='part',
+            name='revision',
+        ),
+    ]