create "3dstreet takeout" service to assist with migration

[kfarr]

With upcoming breaking changes to the way that we deal with user data, we will need a mechanism to export all data for a given user, and then to run this for all users and share the output with them.

Message for end-users (draft):
Thanks to our users for providing excellent feedback on new features for 3DStreet Editor. To implement these new features, we will need to introduce breaking changes to the way that we store information about scenes. In other words, the new updates will not reliably edit old scene data.

As 3DStreet is a free service it is difficult for us to spend labor time to support migration of old scenes, but we realize you don't want to lose your data representing the awesome 3DStreet scenes you've made before. So we're proposing a "3DStreet Takeout" service that will compile a .zip file of all of your 3DStreet scenes. All of your existing scenes will be available there, and you can open them in your browser.

Prompt for task assistance:

• create a script for account "take-out" service to facilitate the user facing text copy provided, ideally as a node.js script that can be run from the command line such as npm run takeout
• this is using google cloud platform firebase firestore as the database server
• this script should, for each user id in google auth sevice:
• given userid, for each scene in scenes collection with that userid as author field value, export the scene data as a json file in a temp directory and name the json file after the field title if exists otherwise use the document id. Combine all the files into a zip file for that user-id. Name each zip file as the user id.

Which items would be breaking changes that are coming up?

• save / save as bug fix -- not breaking but tough to fix without resetting db (ie resetting db is the solution)
• new environment presets -- might not be breaking?
• streetmix parser changes -- not breaking per se but new files will be different center position than old files