config refactor (#1854)

* split out config

* add tests for base and app config, refactor client config out of app config

* refactor default config retrieval

* create config test class and helper functions

* move default_config into server to fix import issue

server/db/cellxgene_orm.py

@@ -1,11 +1,6 @@
 import uuid
 
-from sqlalchemy import (
-    Column,
-    DateTime,
-    ForeignKey,
-    String,
-    func, JSON)
+from sqlalchemy import Column, DateTime, ForeignKey, String, func, JSON
 from sqlalchemy.dialects.postgresql import UUID
 from sqlalchemy.ext.declarative import declarative_base
 from sqlalchemy.orm import relationship