Add feature guard for custom emoji

Custom Emoji is still behind a default-disabled feature flag [1]. With
this change a few codepaths that weren't guarded by the feature flag are
now disabled when the feature is not active.

1. https://gitlab.com/gitlab-org/gitlab/-/issues/231317

app/models/custom_emoji.rb

@@ -36,7 +36,7 @@ class CustomEmoji < ApplicationRecord
 
     namespace = resource.root_ancestor
 
-    return none if namespace.nil?
+    return none if namespace.nil? || Feature.disabled?(:custom_emoji, namespace)
 
     namespace.custom_emoji
   end

spec/models/award_emoji_spec.rb

@@ -277,5 +277,17 @@ RSpec.describe AwardEmoji do
 
       expect(new_award.url).to eq(custom_emoji.url)
     end
+
+    context 'feature flag disabled' do
+      before do
+        stub_feature_flags(custom_emoji: false)
+      end
+
+      it 'does not query' do
+        new_award = build_award(custom_emoji.name)
+
+        expect(ActiveRecord::QueryRecorder.new { new_award.url }.count).to be_zero
+      end
+    end
   end
 end