Implement payment backends and interaction

Advances #3 (closed)

aleksis/apps/tezor/models/base.py

@@ -11,3 +11,6 @@ class Client(ExtensibleModel):
         constraints = [
             models.UniqueConstraint(fields=["name", "site"], name="uniq_client_per_site")
         ]
+
+    def __str__(self) -> str:
+        return self.name