about.gitlab.com` host to variable

Refactor these files URLs from https://about.gitlab.com to promo_host, allowing for JiHu to rewrite it to https://about.gitlab.cn.

app/assets/javascripts/alerts_settings/components/alerts_settings_form.vue

app/helpers/projects/security/configuration_helper.rb

app/views/help/index.html.haml

ee/app/assets/javascripts/admin/subscriptions/show/components/subscription_activation_form.vue

ee/app/assets/javascripts/admin/subscriptions/show/constants.js

ee/app/views/admin/licenses/new.html.haml

ee/app/views/shared/billings/_billing_plan.html.haml