Show button to purchase extra CI minutes only for paid plans

The ability to purchase extra CI minutes is only available for
Namespaces with a paid plan for now. In order to avoid confusion
we should hide this button when the Namespace is using a Free plan.

ee/app/models/ee/namespace.rb

@@ -284,6 +284,10 @@ module EE
       actual_plan_name == GOLD_PLAN
     end
 
+    def paid_plan?
+      !(free_plan? || early_adopter_plan?)
+    end
+
     def use_elasticsearch?
       ::Gitlab::CurrentSettings.elasticsearch_indexes_namespace?(self)
     end

ee/app/views/groups/pipeline_quota/index.html.haml

@@ -10,9 +10,10 @@
     %strong= @group.name
     group
 
-  .col-sm-6
-    %p.text-right
-      = link_to 'Buy additional minutes', 'https://customers.gitlab.com/subscriptions', target: '_blank', class: 'btn btn-inverted btn-success right text-right'
+  - if @group.paid_plan?
+    .col-sm-6
+      %p.text-right
+        = link_to 'Buy additional minutes', 'https://customers.gitlab.com/subscriptions', target: '_blank', class: 'btn btn-inverted btn-success right text-right'
 
 = render "namespaces/pipelines_quota/list",
   locals: { namespace: @group, projects: @projects }

ee/spec/models/ee/namespace_spec.rb

@@ -56,4 +56,36 @@ describe Namespace do
       expect(namespace.use_elasticsearch?).to eq(true)
     end
   end
+
+  describe '#paid_plan?' do
+    using RSpec::Parameterized::TableSyntax
+
+    let(:namespace) { create(:namespace) }
+
+    before(:all) do
+      %i[free_plan early_adopter_plan bronze_plan silver_plan gold_plan].each do |plan|
+        create(plan)
+      end
+    end
+
+    subject { namespace.paid_plan? }
+
+    where(:plan_code, :expected_result) do
+      described_class::FREE_PLAN          | false
+      described_class::EARLY_ADOPTER_PLAN | false
+      described_class::BRONZE_PLAN        | true
+      described_class::SILVER_PLAN        | true
+      described_class::GOLD_PLAN          | true
+    end
+
+    with_them do
+      before do
+        namespace.update!(gitlab_subscription_attributes: { hosted_plan: Plan.find_by_name(plan_code) })
+      end
+
+      it do
+        is_expected.to eq(expected_result)
+      end
+    end
+  end
 end