move financial deets to an environment variable

lets us keep cabinet office financials safe in the credentials repo

the dict in the creds repo will either be an empty dict or a full dict,
so the env var on paas will always contain some parseable json. But
locally it might not, so if it's not set at all then default to the
string `null` so the json parsing doesn't throw a wobbly.

app/main/views/index.py

@@ -1,5 +1,6 @@
 from flask import (
     abort,
+    current_app,
     make_response,
     redirect,
     render_template,
@@ -86,6 +87,7 @@ def billing_details():
     if current_user.is_authenticated:
         return render_template(
             'views/pricing/billing-details.html',
+            billing_details=current_app.config['NOTIFY_BILLING_DETAILS'],
             navigation_links=pricing_nav(),
         )
     return render_template(