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`.

app/serializers/listable_topic_serializer.rb

@@ -111,7 +111,7 @@ class ListableTopicSerializer < BasicTopicSerializer
   alias :include_new_posts? :has_user_data
 
   def include_excerpt?
-    pinned || SiteSetting.always_include_topic_excerpts
+    pinned || SiteSetting.always_include_topic_excerpts || ThemeModifierHelper.new(request: scope.request).serialize_topic_excerpts
   end
 
   def pinned