Implement auto-rounding decimal field

Ref: https://stackoverflow.com/questions/37958130/automatically-round-djangos-decimalfield-according-to-the-max-digits-and-decima

InvenTree/company/migrations/0011_auto_20200318_1114.py

@@ -0,0 +1,20 @@
+# Generated by Django 2.2.9 on 2020-03-18 11:14
+
+import InvenTree.fields
+import django.core.validators
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('company', '0010_auto_20200201_1231'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='supplierpricebreak',
+            name='cost',
+            field=InvenTree.fields.RoundingDecimalField(decimal_places=5, max_digits=10, validators=[django.core.validators.MinValueValidator(0)]),
+        ),
+    ]

InvenTree/company/migrations/0012_auto_20200318_1114.py

@@ -0,0 +1,20 @@
+# Generated by Django 2.2.9 on 2020-03-18 11:14
+
+import InvenTree.fields
+import django.core.validators
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('company', '0011_auto_20200318_1114'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='supplierpricebreak',
+            name='quantity',
+            field=InvenTree.fields.RoundingDecimalField(decimal_places=5, default=1, max_digits=15, validators=[django.core.validators.MinValueValidator(1)]),
+        ),
+    ]