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

@shane-tomlinson feedback appreciated!
`afterCompleteSignUp` is a duplicate, as is `afterCompleteResetPassword` but it appears there's a [bug in the original test](https://github.com/mozilla/fxa-content-server/pull/1987/files#diff-0c50d3314fac349e8ff2d15330573c14R164). It's calling `afterSignIn` rather than `afterCompleteResetPassword`.