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

app/authentication/utils.py

@@ -0,0 +1,12 @@
+from flask import current_app
+from itsdangerous import URLSafeSerializer
+
+
+def get_secret(secret):
+    serializer = URLSafeSerializer(current_app.config.get('SECRET_KEY'))
+    return serializer.loads(secret, salt=current_app.config.get('DANGEROUS_SALT'))
+
+
+def generate_secret(token):
+    serializer = URLSafeSerializer(current_app.config.get('SECRET_KEY'))
+    return serializer.dumps(str(token), current_app.config.get('DANGEROUS_SALT'))