unify auto-signup and regular accounts

[phritz]

this is a necessary first step to hooking auto-signup accounts properly into the diffserver

• move hardcoded accounts from serve/accounts to acccount/
• present a unified account.Records interface that includes both auto-signup and regular accounts
• remove special case for unittest account, leaving only one special case ("sandbox") where the authorization header sent is not an account id
• serve.Service now depends on account.DB instead of an array of Accounts

@aboodman you can pull this branch, add commits, and push (or even force push) if you want. I DO NOT want to merge it until am there watching after the merge to make sure production is happy.

progress towards rocicorp/repc#269

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[phritz]

@aboodman merged this so i can keep working