Abstract the PartAttachment class

Now "Attachments" are much easier to implement for different models

InvenTree/part/migrations/0032_auto_20200322_0453.py

@@ -0,0 +1,19 @@
+# Generated by Django 2.2.9 on 2020-03-22 04:53
+
+import InvenTree.models
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('part', '0031_auto_20200318_1044'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='partattachment',
+            name='attachment',
+            field=models.FileField(help_text='Select file to attach', upload_to=InvenTree.models.rename_attachment),
+        ),
+    ]