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

...e email

fixes #1621

@ckarlof r?
> he problem here is that we need a way to signal to the functional tests that sign-in was successful,

From irc: I believe the desktop context also sends an event when it is done (just like the WebChannel) so we can probably use https://github.com/mozilla/fxa-content-server/blob/master/tests/functional/oauth_webchannel.js#L151to detect a successful flow