add POST letter schema

similar to sms/email, however, for consistency with response values
and internal storage, rather than supplying an "email_address" field
or a "phone_number" field, supply an "address_line_1" and "postcode"
field within the personalisation object.

app/schema_validation/definitions.py

@@ -20,6 +20,9 @@ personalisation = {
 }
 
 
+letter_personalisation = dict(personalisation, required=["address_line_1", "postcode"])
+
+
 https_url = {
     "type": "string",
     "format": "uri",