Refactor czi_hosted and server into backend directory, pull common code into backend/common, refactor tests (#2102)

* move local_server -> backend/server server-> backend/czi_hosted, pull common code into backend/common update imports, tests and make commands

backend/server/__init__.py

@@ -0,0 +1,13 @@
+import logging
+import sys
+from backend.common.utils.utils import import_plugins
+
+__version__ = "0.16.0"
+display_version = "cellxgene v" + __version__
+
+try:
+    import_plugins("backend.server.plugins")
+except Exception as e:
+    # Make sure to exit in this case, as the server may not be configured as expected.
+    logging.critical(f"Error in import_plugins: {str(e)}")
+    sys.exit(1)