This commit changes the custom flags admin config area
to use the AdminPageHeader component and conforms
to the new admin UI guidelines. Also makes some slight
modifications to the reusable components to handle using
a @route for an action button.
Admin can create up to 50 custom flags. It is limited for performance reasons.
When the limit is reached "Add button" is disabled and backend is protected by guardian.
* `@ember/owner` instead of `@ember/application`
* `discourse-i18n` instead of `I18n`
* `{ service } from "@ember/service"` instead of `inject as service`
