Add the InvenTreeSetting model

- Storage of singleton settings in key:value pairs

InvenTree/common/migrations/0004_inventreesetting.py

@@ -0,0 +1,21 @@
+# Generated by Django 2.2.5 on 2019-09-15 12:44
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('common', '0003_auto_20190902_2310'),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='InvenTreeSetting',
+            fields=[
+                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('key', models.CharField(help_text='Settings key', max_length=50, unique=True)),
+                ('value', models.CharField(blank=True, help_text='Settings value', max_length=200)),
+            ],
+        ),
+    ]