Add models for manual invoicing

Advances #2

aleksis/apps/tezor/migrations/0002_invoice_due_date.py

+# Generated by Django 3.2.12 on 2022-03-12 21:25
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('tezor', '0001_initial'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='invoice',
+            name='due_date',
+            field=models.DateField(null=True, verbose_name='Payment due date'),
+        ),
+    ]