Bug 1047521 - Set up a way for people to create webmaker accounts from welcome.wmo

<https://bugzilla.mozilla.org/show_bug.cgi?id=1047521>
This can be removed safely, we default it to the right thing in [webmaker-auth](https://github.com/mozilla/webmaker-auth-client/blob/2c0eb1f6bcf2673667e78a242517c026e5ab54d1/webmaker-auth-client.js#L52)