Add "whats new" item to help dropdown

Added a "whats new at GitLab" item to the help dropdown. Also added a
helper to retreive the plan name if a project exits.

part of: https://gitlab.com/gitlab-org/growth/product/-/issues/1487

app/views/layouts/header/_help_dropdown.html.haml

 %ul
   - if current_user_menu?(:help)
+    = render_if_exists 'layouts/header/whats_new_dropdown_item'
     %li
       = link_to _("Help"), help_path
     %li

ee/app/helpers/ee/application_helper.rb

@@ -117,6 +117,10 @@ module EE
       allow_access_to_onboarding?
     end
 
+    def show_whats_new_dropdown_item?
+      ::Gitlab.com? && ::Feature.enabled?(:whats_new_dropdown)
+    end
+
     private
 
     def appearance

ee/app/views/layouts/header/_whats_new_dropdown_item.html.haml

+- if show_whats_new_dropdown_item?
+  %li
+    = link_to _("See what's new at GitLab"), "#{promo_url}/releases/categories/releases/", target: '_blank', rel: 'noopener noreferrer', data: { track_event: 'click_whats_new', track_property: 'question_menu' }

ee/changelogs/unreleased/jswain_whats_new.yml

+---
+title: Add "whats new" item to help dropdown
+merge_request: 29183
+author:
+type: added

ee/spec/helpers/application_helper_spec.rb

@@ -187,4 +187,26 @@ describe ApplicationHelper do
       end
     end
   end
+
+  describe '#show_whats_new_dropdown_item?' do
+    using RSpec::Parameterized::TableSyntax
+
+    subject { helper.show_whats_new_dropdown_item? }
+
+    where(:feature_flag, :gitlab_com, :result) do
+      true  | true  | true
+      true  | false | false
+      false | true  | false
+      false | false | false
+    end
+
+    with_them do
+      before do
+        stub_feature_flags(whats_new_dropdown: feature_flag)
+        allow(::Gitlab).to receive(:com?).and_return(gitlab_com)
+      end
+
+      it { is_expected.to be(result) }
+    end
+  end
 end

locale/gitlab.pot

@@ -18168,6 +18168,9 @@ msgstr ""
 msgid "See the affected projects in the GitLab admin panel"
 msgstr ""
 
+msgid "See what's new at GitLab"
+msgstr ""
+
 msgid "Select"
 msgstr ""