Publish a Swagger specification

Adds a new endpoint (`/spec`) which returns a the specification of the
API in Swagger-formatted JSON.

This means we will have something to point frontends at, so we can
evaluate which ones we like.

Right now it’s all hand-defined. If we were consistent about our use of
Marshmallow we could generated the spec from the Marshmallow schemas.

tests/app/spec/test_rest.py

@@ -0,0 +1,18 @@
+import flex
+import pytest
+
+from flask import json
+from tests import create_authorization_header
+
+
+def test_spec_returns_valid_json(notify_api, sample_notification):
+    with notify_api.test_request_context():
+        with notify_api.test_client() as client:
+            auth_header = create_authorization_header(service_id=sample_notification.service_id)
+
+            response = client.get('/spec', headers=[auth_header])
+
+            # Check that it’s a valid Swagger schema
+            flex.load(
+                json.loads(response.get_data(as_text=True))
+            )