EE: Move EE specific code into ee/ dir

Restore app/helpers/clusters_helper.rb to CE, move override to EE
module.

app/helpers/clusters_helper.rb

 # frozen_string_literal: true
 
 module ClustersHelper
+  prepend EE::ClustersHelper
+
+  # EE overrides this
   def has_multiple_clusters?
-    clusterable.feature_available?(:multiple_clusters)
+    false
   end
 
   def render_gcp_signup_offer

ee/app/helpers/ee/clusters_helper.rb

+# frozen_string_literal: true
+
+module EE
+  module ClustersHelper
+    extend ::Gitlab::Utils::Override
+
+    override :has_multiple_clusters?
+    def has_multiple_clusters?
+      clusterable.feature_available?(:multiple_clusters)
+    end
+  end
+end

ee/spec/helpers/ee/clusters_helper_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe ClustersHelper do
+  describe '#has_multiple_clusters?' do
+    let(:project) { build(:project) }
+
+    subject { helper.has_multiple_clusters? }
+
+    before do
+      # clusterable is provided as a `helper_method`
+      allow(helper).to receive(:clusterable).and_return(project)
+    end
+
+    context 'license is premium' do
+      before do
+        expect(project).to receive(:feature_available?).with(:multiple_clusters).and_return(true)
+      end
+
+      it { is_expected.to be_truthy }
+    end
+
+    context 'license is starter' do
+      before do
+        expect(project).to receive(:feature_available?).with(:multiple_clusters).and_return(false)
+      end
+
+      it { is_expected.to be_falsey }
+    end
+  end
+end