Use json schema to validate post request to send PDF letter

This approach was suggested as the way to do validation things on
Notify.

app/service/send_pdf_letter_schema.py

@@ -0,0 +1,14 @@
+send_pdf_letter_request = {
+    "$schema": "http://json-schema.org/draft-04/schema#",
+    "description": "POST send uploaded pdf letter",
+    "type": "object",
+    "title": "Send an uploaded pdf letter",
+    "properties": {
+        "postage": {"enum": ["first", "second"]},
+        "filename": {"type": "string"},
+        "created_by": {"type": "string"},
+        "file_id": {"type": "string"},
+    },
+
+    "required": ["postage", "filename", "created_by", "file_id"]
+}