Add back icons to components once Gitlab UI buttons support icons through props

!18409 (closed) removes two icons that are blocking a change that will allow our modals to strictly follow the design system. The icons removed are within the two components below and can be found in the diff

1. ee/app/assets/javascripts/vue_merge_request_widget/components/approvals/approvals_auth.vue
2. ee/app/assets/javascripts/vue_merge_request_widget/components/visual_review_app_link.vue

Once gitlab-ui button components support icons through props we should revisit the components listed above and support the icons through props to the gitlab-ui button component.