[RUN AS-IF-FOSS] Set suggestion_path variable for non-trial registrations

What does this MR do?

This fixes the errors with non-EE distributions as reported here: #282470 (closed)

The app/views/devise/shared/_signup_box.html.haml partial waas made to be shared by the app/views/devise/registrations/new.html.haml and ee/app/views/trial_registrations/new.html.haml templates in this MR. This MR sets the suggestion_path variable to nil for the former, like it was before the recent changes.