Introduce a config file to cellxgene (#1264)

* Introduce a config file to cellxgene

The config file format is in yaml.  The default config is located
in server/common/default_config.py.  A user may create a yaml file
that contains a subset of these fields.  It can be used during cellxgene
launch, or for hosted cellxgene.

The code has also been refactored.  Much of the logic to check arguments
has moved from launch to app config.

It is now possible to set the tiledb context parameters using the config
file.  Other feature will soon be handled in a similar way.

server/common/rest.py

@@ -33,7 +33,7 @@ def schema_get(data_adaptor, annotations):
 
 
 def config_get(app_config, data_adaptor, annotations):
-    config = app_config.get_config(data_adaptor, annotations)
+    config = app_config.get_client_config(data_adaptor, annotations)
     return make_response(jsonify(config), HTTPStatus.OK)
 
 
@@ -131,7 +131,7 @@ def data_var_put(request, data_adaptor):
 
 
 def diffexp_obs_post(request, data_adaptor):
-    if data_adaptor.config.disable_diffexp:
+    if not data_adaptor.config.diffexp__enable:
         return make_response(f"diffexp not supported.", HTTPStatus.BAD_REQUEST)
 
     args = request.get_json()
@@ -198,7 +198,7 @@ def layout_obs_put(request, data_adaptor):
     preferred_mimetype = request.accept_mimetypes.best_match(["application/octet-stream"])
     if preferred_mimetype != "application/octet-stream":
         return make_response(f"Unsupported MIME type '{request.accept_mimetypes}'", HTTPStatus.NOT_ACCEPTABLE)
-    if not data_adaptor.config.enable_reembedding:
+    if not data_adaptor.config.embedding__enable_reembedding:
         return make_response(f"Computed embedding not supported.", HTTPStatus.BAD_REQUEST)
 
     args = request.get_json()