Persistent rooms via Firebase (#2188)

* Periodically back up collaborative rooms in firebase

* Responses to code review

* comments from code review, new firebase credentials

firebase-project/firebase.json

@@ -0,0 +1,6 @@
+{
+  "firestore": {
+    "rules": "firestore.rules",
+    "indexes": "firestore.indexes.json"
+  }
+}