Split out the local backend (#2052)

This splits the backend into two parts: the local backend for desktop cellxgene and the AWS backend for hosted cellxgene. The local backend is in local_server while the hosted remains in server. The general idea is to copy everything from server to local_server, pull unneeded stuff out of local_server, and keep server as-is for this PR. Not touching server means all the infra and deployment code will continue working just as it did before so we can make those changes incrementally.

local_server/__main__.py

@@ -0,0 +1,15 @@
+# Work around bug https://github.com/pallets/werkzeug/issues/461
+if __package__ is None:
+    import sys
+    from pathlib import Path
+
+    PKG_PATH = Path(__file__).parent
+    sys.path.insert(0, str(PKG_PATH.parent))
+    import server  # noqa F401
+
+    __package__ = PKG_PATH.name
+
+# Main thing
+from .cli.cli import cli  # noqa F402
+
+cli()