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/cli/launch.py

@@ -275,6 +275,7 @@ class CliLaunchServer(Server):
     """
     the CLI runs a local web server, and needs to enable a few more features.
     """
+
     def __init__(self, matrix_data_cache_manager, annotations, app_config):
         super().__init__(matrix_data_cache_manager, annotations, app_config)
 
@@ -394,6 +395,10 @@ def launch(
 
         app_config.complete_config(matrix_data_cache_manager, messagefn)
 
+        # Use a default secret if one is not provided
+        if not app_config.server__flask_secret_key:
+            app_config.server__flask_secret_key = "SparkleAndShine"
+
     except (ConfigurationError, DatasetAccessError) as e:
         raise click.ClickException(e)