Scrolled themes depend on Sprockets files

[tf]

Currently when adding a theme for Pageflow Scrolled, we still need to provice an SCSS stylesheet, logo and preview images (app/assets/stylesheets/pageflow/themes/<theme_name>.scss, app/assets/images/pageflow/themes/<theme_name>/logo_print.png, app/assets/images/pageflow/themes/<theme_name>/preview{,_thumbnail}.png) in Sprockets since the editor seed data and the structured data helper still depend on those. The Pageflow engine also registers unused theme stylesheets for asset pipeline precompilation.

We need to consider ways to decouple this further.