Added a config hook for secret key into the app. (#1304)

* Added a config hook for secret key into the app.

the server first looks in an environment variable,
then looks in a config file.

For the cellxgene launch app, a default key is used if none is provided.
For the eb app, a secret key must be provided.

server/common/default_config.py

@@ -13,6 +13,7 @@ server:
   about_legal_tos: null
   about_legal_privacy: null
   force_https: false
+  flask_secret_key: null
 
 presentation:
   max_categories: 1000