Switch to using a locally generated primary user id

[specious]

Currently, when someone signs in for their first time (which is with Google, as that is at the moment the only way to sign in), their identity gets created in the users table.

Their Google identifier (prepended by g:) is used as their primary identification key in the system.

It would be proper to generate a sensible internal identifier to use as the primary key that distinguishes them from all other accounts, and link their external accounts to it.

[specious]

On the other hand, it might be quite reasonable to identify the user by the first third-party identity that they signed in with. For sign-in with Facebook and Apple their id could start with f: and a:, respectively.

Their other identities can be linked to the primary one.

If any problems arise with this, migration to a different scheme could be considered.