Implement payment backends and interaction

Advances #3 (closed)

aleksis/apps/tezor/migrations/0001_initial.py

@@ -85,7 +85,7 @@ class Migration(migrations.Migration):
         ),
         migrations.AddConstraint(
             model_name='invoice',
-            constraint=models.UniqueConstraint(fields=('transaction_id', 'group'), name='number_uniq_per_group'),
+            constraint=models.UniqueConstraint(fields=('number', 'group'), name='number_uniq_per_group'),
         ),
         migrations.AddConstraint(
             model_name='client',