DEV: Use separate files for theme component stylesheets (#12214)

This switches to outputting a separate file for each theme component CSS
asset. We have separate CSS plugin files, separate JS files
(for plugins/themes/components), it makes sense to do the same for
component CSS assets.

Benefits:
- easier debugging
- fixes a regression with theme component sourcemaps
- changes to theme components are updated individually

With HTTP/2, there is also no performance downside to having additional
files in the initial request.

lib/stylesheet/importer.rb

@@ -194,18 +194,13 @@ module Stylesheet
       fields.map do |field|
         value = field.value
         if value.present?
-          filename = "theme_#{field.theme.id}/#{field.target_name}-#{field.name}-#{field.theme.name.parameterize}.scss"
           contents << <<~COMMENT
           // Theme: #{field.theme.name}
           // Target: #{field.target_name} #{field.name}
           // Last Edited: #{field.updated_at}
           COMMENT
 
-          if field.theme_id == theme.id
-            contents << value
-          else
-            contents << field.compiled_css(prepended_scss)
-          end
+          contents << value
         end
 
       end

lib/stylesheet/manager.rb

@@ -58,7 +58,7 @@ class Stylesheet::Manager
 
     theme_ids = [theme_ids] unless Array === theme_ids
     theme_ids = [theme_ids.first] unless target =~ THEME_REGEX
-    theme_ids = Theme.transform_ids(theme_ids, extend: false)
+    theme_ids = Theme.transform_ids(theme_ids)
 
     current_hostname = Discourse.current_hostname