AccountService: Implements default account failsafe

Implements one of the fixes outlined in issue #2637.

/cc @sendhil + @astralbodies + @koke 

Is there any particular reason why we use a locally generated UUID instead of using the account's unique ID (which I believe is the username)?

I understand that this may be a very different point of view on how to implement this, so please bear with me! :disappointed_relieved: