Sign-up in CSS with SolidOS/Mashlib recipe is unintuitive

[renyuneyun]

(I'm not sure whether this belong to this repo or the mashlib repo, but it's unlikely an issue with CSS-recipe repo.)

When using CSS with the Mashlib recipe, the visitor will be given a UI with login and sign up buttons.

But problem is, the "sign-up" button will redirect you to solidproject.org, not to the sign-up page of the current CSS instance.
The only way to sign up (from UI) is to click sign-in, and then click the small sign-up link in the new page.

This really makes things complicated for new users, if the server is using CSS and mashlib.