Hide "Buy additional minutes" button for self-managed installs

This button only pertains to GitLab.com, so it should not be seen
by other installations.

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/12864

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

@@ -16,7 +16,7 @@
       %a.nav-link#storage-quota{ data: { toggle: "tab", action: '#storage-quota-tab' }, href: '#storage-quota-tab', 'aria-controls': '#storage-quota-tab', 'aria-selected': false }
         = s_('UsageQuota|Storage')
   .nav-controls
-    = link_to s_('UsageQuota|Buy additional minutes'), EE::SUBSCRIPTIONS_PLANS_URL, target: '_blank', class: 'btn btn-inverted btn-success float-right'
+    = link_to s_('UsageQuota|Buy additional minutes'), EE::SUBSCRIPTIONS_PLANS_URL, target: '_blank', class: 'btn btn-inverted btn-success float-right' if ::Gitlab.com?
 
 .tab-content
   .tab-pane#pipelines-quota-tab

ee/changelogs/unreleased/sh-hide-additional-minutes.yml

+---
+title: Hide "Buy additional minutes" button for self-managed installs
+merge_request: 14826
+author:
+type: fixed

ee/spec/features/groups/usage_quotas_spec.rb

@@ -4,8 +4,11 @@ describe 'Groups > Usage Quotas' do
   let(:user) { create(:user) }
   let(:group) { create(:group) }
   let!(:project) { create(:project, namespace: group, shared_runners_enabled: true) }
+  let(:gitlab_dot_com) { true }
 
   before do
+    allow(Gitlab).to receive(:com?).and_return(gitlab_dot_com)
+
     group.add_owner(user)
     sign_in(user)
   end
@@ -111,9 +114,21 @@ describe 'Groups > Usage Quotas' do
 
     include_examples 'linked in group settings dropdown'
 
+    context 'when it is not GitLab.com' do
+      let(:gitlab_dot_com) { false }
+
+      it "does not show 'Buy additional minutes' button" do
+        visit_pipeline_quota_page
+
+        expect(page).not_to have_content('Buy additional minutes')
+      end
+    end
+
     it 'shows correct group quota and projects info' do
       visit_pipeline_quota_page
 
+      expect(page).to have_content('Buy additional minutes')
+
       page.within('.pipeline-quota') do
         expect(page).to have_content("1000 / 500 minutes")
         expect(page).to have_content("200% used")