Create anonymous users for not-logged-in visitors

[kasbah]

Firebase has support for anonymous auth allowing us to create temporary anonymous accounts for visitors that are not logged in.

• This will help us to clean up our ownership model, remove the .anyone = Own property, and ensure not-logged-in users can see their own docs (e.g. analyses of public models) while not being overwhelmed by a list of all docs any non-logged-in user has ever created
• Additionally this will allow anon users to create documents, save them and they then continue to be their documents when they sign up.

Possible pit-falls:

• We should delete anon users (but not their docs) after 30 days of inactivity to avoid them piling up. Might be easiest to enable auto-cleanup but we need to be on the "Firebase Authentication with Identity Platform" tier for that (are we?). The docs should probably be marked as orphaned somehow when the user is deleted.
• We should warn anon users they may lose access to their docs and prompt them to sign up.

[epatters]

Nice idea! I didn't know about this feature of Firebase Auth. It sounds better than our current approach. While it would complicate some things (needing to sign in anonymously when appropriate), it should actually simplify the core logic around ownership.

Might be easiest to enable auto-cleanup but we need to be on the "Firebase Authentication with Identity Platform" tier for that (are we?).

We are currently on the free tier, but at the scale that we're currently operating, upgrading to a paid tier should be no big deal.