Add "revision" number to report model

- Auto-increments whenever the model is saved

InvenTree/report/migrations/0009_testreport_revision.py

@@ -0,0 +1,18 @@
+# Generated by Django 3.0.7 on 2021-02-05 00:53
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('report', '0008_auto_20210204_2100'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='testreport',
+            name='revision',
+            field=models.PositiveIntegerField(default=1, help_text='Report revision number (auto-increments)', verbose_name='Revision'),
+        ),
+    ]

InvenTree/report/migrations/0010_auto_20210205_1201.py

@@ -0,0 +1,18 @@
+# Generated by Django 3.0.7 on 2021-02-05 01:01
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('report', '0009_testreport_revision'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='testreport',
+            name='revision',
+            field=models.PositiveIntegerField(default=1, editable=False, help_text='Report revision number (auto-increments)', verbose_name='Revision'),
+        ),
+    ]