Add _links.self to Release API endpoints

This commit adds a new property, _links.self, to two Release API
endpoints.

app/presenters/release_presenter.rb

@@ -19,6 +19,12 @@ class ReleasePresenter < Gitlab::View::Presenter::Delegated
     project_tag_path(project, release.tag)
   end
 
+  def self_url
+    return unless ::Feature.enabled?(:release_show_page, project)
+
+    project_release_url(project, release)
+  end
+
   def merge_requests_url
     return unless release_mr_issue_urls_available?
 

lib/api/entities.rb

@@ -1101,6 +1101,7 @@ module API
         expose :evidence_file_path, expose_nil: false, if: ->(_, _) { can_download_code? }
       end
       expose :_links do
+        expose :self_url, as: :self, expose_nil: false
         expose :merge_requests_url, expose_nil: false
         expose :issues_url, expose_nil: false
         expose :edit_url, expose_nil: false

spec/presenters/release_presenter_spec.rb

@@ -51,6 +51,22 @@ describe ReleasePresenter do
     end
   end
 
+  describe '#self_url' do
+    subject { presenter.self_url }
+
+    it 'returns its own url' do
+      is_expected.to match /#{project_release_url(project, release)}/
+    end
+
+    context 'when release_show_page feature flag is disabled' do
+      before do
+        stub_feature_flags(release_show_page: false)
+      end
+
+      it { is_expected.to be_nil }
+    end
+  end
+
   describe '#merge_requests_url' do
     subject { presenter.merge_requests_url }