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/test/fixtures/fixtures.py

@@ -0,0 +1,12 @@
+pbmc3k_colors = {
+    "louvain": {
+        "B cells": "#2ca02c",
+        "CD14+ Monocytes": "#ff7f0e",
+        "CD4 T cells": "#1f77b4",
+        "CD8 T cells": "#d62728",
+        "Dendritic cells": "#e377c2",
+        "FCGR3A+ Monocytes": "#8c564b",
+        "Megakaryocytes": "#bcbd22",
+        "NK cells": "#9467bd",
+    }
+}