[bug-861360] Displaying the signed in user data from the UI

https://bugzilla.mozilla.org/show_bug.cgi?id=861360

This ensures that we show the user displayname/subdomain for our users once they are signed in - something we didn't have for all use cases before.

This gives us:

* logged in users returning
* logged in users who already have a webmaker account on a secondary page
* users without an existing subdomain/displayName