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/common/utils/corpora_constants.py

@@ -0,0 +1,22 @@
+class CorporaConstants(object):
+    REQUIRED_SIMPLE_METADATA_FIELDS = [
+        "version",
+        "title",
+        "layer_descriptions",
+        "organism",
+        "organism_ontology_term_id",
+    ]
+
+    # The Corpora specification requires some values encoded as JSON due to the inability of AnnData to store complex
+    # types.
+    OPTIONAL_JSON_ENCODED_METADATA_FIELD = ["contributors", "project_links"]
+
+    OPTIONAL_SIMPLE_METADATA_FIELDS = [
+        "preprint_doi",
+        "publication_doi",
+        "default_embedding",
+        "default_field",
+        "tags",
+        "project_name",
+        "project_description",
+    ]