banzai: Get Custom Emoji from group as well

Not all markdown fields are related to a project. Some, like those in
Epics, might be related to a group. So in addition to finding the
containing namespace through the project, also get try to get it from
the group or namespace.

Because we moved the responsibility of looking up the root namespace to
the scope #for_resource in CustomEmoji, we can rely on that.

This fixes custom emoji in Epic descriptions and notes.

Changelog: fixed

lib/banzai/filter/custom_emoji_filter.rb

@@ -8,8 +8,7 @@ module Banzai
       IGNORED_ANCESTOR_TAGS = %w(pre code tt).to_set
 
       def call
-        return doc unless context[:project]
-        return doc unless Feature.enabled?(:custom_emoji, context[:project])
+        return doc unless resource_parent
 
         doc.xpath('descendant-or-self::text()').each do |node|
           content = node.to_html
@@ -50,12 +49,12 @@ module Banzai
 
       def has_custom_emoji?
         strong_memoize(:has_custom_emoji) do
-          namespace&.custom_emoji&.any?
+          CustomEmoji.for_resource(resource_parent).any?
         end
       end
 
-      def namespace
-        context[:project].namespace.root_ancestor
+      def resource_parent
+        context[:project] || context[:group]
       end
 
       def custom_emoji_candidates
@@ -63,7 +62,8 @@ module Banzai
       end
 
       def all_custom_emoji
-        @all_custom_emoji ||= namespace.custom_emoji.by_name(custom_emoji_candidates).index_by(&:name)
+        @all_custom_emoji ||=
+          CustomEmoji.for_resource(resource_parent).by_name(custom_emoji_candidates).index_by(&:name)
       end
     end
   end