API: Expose URL of award emoji image

For custom emoji there is source image URL. This change exposes that
through the award emoji API when it is available.

app/models/award_emoji.rb

@@ -60,6 +60,10 @@ class AwardEmoji < ApplicationRecord
     self.name == UPVOTE_NAME
   end
 
+  def url
+    awardable.try(:namespace)&.custom_emoji&.by_name(name)&.first&.url
+  end
+
   def expire_cache
     awardable.try(:bump_updated_at)
     awardable.try(:expire_etag_cache)

lib/api/entities/award_emoji.rb

@@ -8,6 +8,7 @@ module API
       expose :user, using: Entities::UserBasic
       expose :created_at, :updated_at
       expose :awardable_id, :awardable_type
+      expose :url
     end
   end
 end

spec/requests/api/award_emoji_spec.rb

@@ -26,6 +26,23 @@ RSpec.describe API::AwardEmoji do
         expect(json_response.first['name']).to eq(award_emoji.name)
       end
 
+      it "includes custom emoji attributes" do
+        group = create(:group)
+        group.add_maintainer(user)
+
+        project = create(:project, namespace: group)
+        custom_emoji = create(:custom_emoji, name: 'partyparrot', namespace: group)
+        issue = create(:issue, project: project)
+        create(:award_emoji, awardable: issue, user: user, name: custom_emoji.name)
+
+        get api("/projects/#{project.id}/issues/#{issue.iid}/award_emoji", user)
+
+        expect(response).to have_gitlab_http_status(:ok)
+        expect(json_response).to be_an Array
+        expect(json_response.first['name']).to eq(custom_emoji.name)
+        expect(json_response.first['url']).to eq(custom_emoji.file)
+      end
+
       it "returns a 404 error when issue id not found" do
         get api("/projects/#{project.id}/issues/#{non_existing_record_iid}/award_emoji", user)