Force sass to compile imported files by making .scss extension explicit

[jorg-vr]

This pull request fixes the issue that due to recent updates in sass, @import "xxxxx.css" are no longer compiled.
The specification can be found here: https://sass-lang.com/documentation/at-rules/import#plain-css-imports

The issue manifested itself in our case with the following error:

ActionView::Template::Error (Error: Undefined variable: "$development".
        on line 70:5 of app/assets/stylesheets/theme/theme-light.css.scss
        from line 11:9 of app/assets/stylesheets/base.css.scss
>> @if $development == true {

   ----^
):

Which is fixed by this pr