FIX: Hide broken theme about/license URLs (#29930)

At the top of the theme show page we have a link
to the theme About and License, which are supposed
to be URLs. However some themes have left placeholder
text in these metadata fields, which leads to a wonky
experience.

Instead, we can just not serialize these fields if they
are not valid URLs, then they will not show as links
in the UI.

app/serializers/remote_theme_serializer.rb

@@ -19,10 +19,14 @@ class RemoteThemeSerializer < ApplicationSerializer
              :minimum_discourse_version,
              :maximum_discourse_version
 
-  # wow, AMS has some pretty nutty logic where it tries to find the path here
-  # from action dispatch, tell it not to
+  # ActiveModelSerializer has some pretty nutty logic where it tries to find
+  # the path here from action dispatch, tell it not to
   def about_url
-    object.about_url
+    object.about_url if UrlHelper.is_valid_url?(object.about_url)
+  end
+
+  def license_url
+    object.license_url if UrlHelper.is_valid_url?(object.license_url)
   end
 
   def include_github_diff_link?