fix(signin): pass account data to broker instead of using currentUser

@shane-tomlinson feedback appreciated!
Thinking about this some more, it may be clearer to break the `_signIn` up - from here down to [the end of this block](https://github.com/mozilla/fxa-content-server/pull/1987/files#diff-ff872bd2bd254614b59baa704fbfd876R129), and move these two paths into `submit` and `useLoggedInAccount`. Once the session is established/verified, call the rest of the `_signIn` function from `return self.user.setCurrentAccount(account)` on down.