Move SupplierPart and SupplierPriceBreak to the 'Company' app

- https://docs.djangoproject.com/en/2.2/ref/models/options/#django.db.models.Options.db_table
- https://stackoverflow.com/questions/3519143/django-how-to-specify-a-database-for-a-model
- And others, presumably

InvenTree/stock/migrations/0017_auto_20190518_1759.py

@@ -0,0 +1,19 @@
+# Generated by Django 2.2 on 2019-05-18 07:59
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('stock', '0016_auto_20190512_2119'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='stockitem',
+            name='supplier_part',
+            field=models.ForeignKey(blank=True, help_text='Select a matching supplier part for this stock item', null=True, on_delete=django.db.models.deletion.SET_NULL, to='company.SupplierPart'),
+        ),
+    ]

InvenTree/stock/models.py

@@ -22,6 +22,7 @@ from InvenTree import helpers
 from InvenTree.models import InvenTreeTree
 
 from part.models import Part
+from company.models import SupplierPart
 
 
 class StockLocation(InvenTreeTree):
@@ -188,7 +189,7 @@ class StockItem(models.Model):
 
     part = models.ForeignKey('part.Part', on_delete=models.CASCADE, related_name='locations', help_text='Base part')
 
-    supplier_part = models.ForeignKey('part.SupplierPart', blank=True, null=True, on_delete=models.SET_NULL,
+    supplier_part = models.ForeignKey(SupplierPart, blank=True, null=True, on_delete=models.SET_NULL,
                                       help_text='Select a matching supplier part for this stock item')
 
     location = models.ForeignKey(StockLocation, on_delete=models.DO_NOTHING,