First pass at implementing API authentication using new JWT tokens

- NOTE - this does not manage secrets. There is only one URL and there is no functionality implemented
- prior to rolling out we need to store secrets properly

Uses the JWT libraries in [https://github.com/alphagov/notifications-python-client](https://github.com/alphagov/notifications-python-client)

- Tokens are checked on every request and will be rejected if token is invalid as per the rules in the python clients.

app/__init__.py

@@ -1,9 +1,12 @@
 import os
 
 from flask._compat import string_types
-from flask import Flask
+from flask import Flask, _request_ctx_stack
+from werkzeug.local import LocalProxy
 from config import configs
 
+api_user = LocalProxy(lambda: _request_ctx_stack.top.api_user)
+
 
 def create_app(config_name):
     application = Flask(__name__)