Implement rollbacks when multisite_account.json fails

[tanius]

We now have an API for remote creation of Discourse accounts on our SSO provider site (docs).

At some point later, rollbacks should be implemented when this mechanism fails to create an account as required. For example, if for some reason one of the requested Admin API keys cannot be created, then all accounts created up to that point have to be removed again without a trace: one on the SSO provider site, one or more on SSO client sites.