move common code into server, update tests and makefile (#2425)

* move common code into server, update tests and makefile

remove backend directory, refactor

update smoke tests

server/app/session.py

@@ -0,0 +1,13 @@
+from uuid import uuid4
+
+from flask.sessions import SessionMixin
+
+CXGUID = "cxguid"
+
+
+def get_user_id(session: SessionMixin) -> str:
+    """ Gets a session-persistent user id. Creates one in the Flask session if non-extant """
+    if CXGUID not in session:
+        session[CXGUID] = uuid4().hex
+        session.permanent = True
+    return session[CXGUID]