FEATURE: Allow themes to specify modifiers in their about.json file (#9097)

There are three modifiers:
- serialize_topic_excerpts (boolean)
- csp_extensions (array of strings)
- svg_icons (array of strings)

When multiple themes are active, the values will be combined. The combination method varies based on the setting. CSP/SVG arrays will be combined. serialize_topic_excerpts will use `Enumerable#any`.

lib/svg_sprite/svg_sprite.rb

@@ -399,6 +399,8 @@ License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL
       end
     end
 
+    theme_icon_settings |= ThemeModifierHelper.new(theme_ids: theme_ids).svg_icons
+
     theme_icon_settings
   end