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

@shane-tomlinson feedback appreciated!
`useLoggedInAccount` should probably also be wrapped with [allowOnlyOneSubmit](https://github.com/mozilla/fxa-content-server/blob/f19779e9e25aeb479171119e5d16abe90e723d82/app/scripts/views/decorators/allow_only_one_submit.js) to ensure only one submit can occur at a time.