Make Whitenoise serve static assets

Currently it’s not configured properly, so isn’t having any effect.
This change makes it wrap the Flask app, so it intercepts any requests
for static content.

Follows the pattern documented in http://whitenoise.evans.io/en/stable/flask.html#enable-whitenoise

application.py

@@ -12,4 +12,4 @@ STATIC_URL = 'static/'
 app = Flask('app')
 
 create_app(app)
-application = WhiteNoise(app, STATIC_ROOT, STATIC_URL)
+app.wsgi_app = WhiteNoise(app.wsgi_app, STATIC_ROOT, STATIC_URL)