Create a migration script from GS 1.0

[DavidGolodetsky]

"Actual:" New version isn't stable yet, but it would be nice to use it already. For that, we need to take safety measures like backing up.

"Expected:" The old GS version firebase db will be migrated to the new mongo db. The script for migration will be run locally, but it's going to live in this repo for backing up purposes.

"Tech notes:" Extract firebase db as json file. Create a node.js script for migration, run locally once, deploy the script for future.