refactor authentication code

moved api_key secret manipulation (generating and getting) into
authentiation/utils, and added a property on the model, to facilitate
easier matching of authenticated requests and the api keys they used

tests/app/authentication/test_utils.py

@@ -0,0 +1,8 @@
+from app.authentication.utils import generate_secret, get_secret
+
+
+def test_secret_is_signed_and_can_be_read_again(notify_api):
+    with notify_api.test_request_context():
+        signed_secret = generate_secret('some_uuid')
+        assert signed_secret != 'some_uuid'
+        assert 'some_uuid' == get_secret(signed_secret)