fix(signin): cache credentials for desktop sign-ins, otherwise only cach...

...e email

fixes #1621

@ckarlof r?
Try this:
```
'sign in on desktop then sign in with prefill does not show picker': function () {
      var self = this;

      return this.get('remote')
        .get(require.toUrl(PAGE_SIGNIN_DESKTOP))
        .findByCssSelector('form input.email')
        .click()
        .type(email)
        .end()

        .findByCssSelector('form input.password')
        .click()
        .type(PASSWORD)
        .end()

        // This will listen for the login event triggered by the submit below
        .then(function () {
          return waitForDesktopLogin(self, PAGE_SIGNIN + '?email=' + email2 );
        })

        .findByCssSelector('button[type="submit"]')
        .click()
        .end()

        .waitForDeletedByCssSelector('button[type="submit"]')
...
```