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/__init__.py

@@ -0,0 +1,11 @@
+import random
+import string
+from os import popen
+
+PROJECT_ROOT = popen("git rev-parse --show-toplevel").read().strip()
+FIXTURES_ROOT = PROJECT_ROOT + "/backend/test/fixtures"
+H5AD_FIXTURE = FIXTURES_ROOT + "/pbmc3k-CSC-gz.h5ad"
+
+
+def random_string(n):
+    return "".join(random.choice(string.ascii_letters) for _ in range(n))