Update Account model

This addresses some of the issues discussed in https://github.com/maiera/gde-app/issues/62

Suggested update approach to prevent breaking the app because of incompatible datastore models.

1. Wipe all accounts from data store
2. Update backend, frontend and GAS-Update-Script
3. Run GAS-Update-Script to fill the Account list again.

/cc @SmokyBob @patt0