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

@shane-tomlinson feedback appreciated!
Would this allow any user that has a session to visit `/confirm`? The redirection to `/confirm` from `/signup` [passes along accountData](https://github.com/mozilla/fxa-content-server/pull/1987/files#diff-af500bac5338814fda67ec4de209ac46R160), we could check for the existence of the passed along `accountData` and use that as the indicator that the user is allowed to see the screen.