Add models for manual invoicing

Advances #2

aleksis/apps/tezor/models/invoice.py

@@ -6,15 +6,14 @@ from django.db.models import Q
 from django.shortcuts import reverse
 from django.utils.translation import gettext_lazy as _
 
-from djmoney.models.fields import CurrencyField, MoneyField
 from payments import PaymentStatus, PurchasedItem
 from payments.models import BasePayment
 
 from aleksis.core.mixins import ExtensibleModel, PureDjangoModel
 from aleksis.core.models import Person
 
-from .base import Client
 from ..tables import PurchasedItemsTable, TotalsTable
+from .base import Client
 
 
 class InvoiceGroup(ExtensibleModel):