Implement notifications service with shared messaging and email drivers (#36)

backend/notifications/templates.example.json

@@ -0,0 +1,15 @@
+[
+  {
+    "id": "exchange.completed",
+    "version": "1",
+    "default_locale": "en",
+    "parameters": ["first_name", "amount", "currency"],
+    "locales": {
+      "en": {
+        "subject": "Your exchange is complete",
+        "html": "<p>Hi {{.first_name}}, your exchange of {{.amount}} {{.currency}} is complete.</p>",
+        "text": "Hi {{.first_name}}, your exchange of {{.amount}} {{.currency}} is complete."
+      }
+    }
+  }
+]