Add merge trains viz feedback banner

Proposal

Add a banner to the merge train page to lead users to the feedback issue: #464774

Technical proposal

Location: SVG is located at https://gitlab-org.gitlab.io/gitlab-svgs/illustrations?q=train and the path is illustrations/train-sm.svg

1. Create a feedback_banner.vue component that uses GlBanner. The button should link to the feedback issue #464774
2. SVG import path should be import MergeTrainSvg from '@gitlab/svgs/dist/illustrations/train-sm.svg?url';
3. Import and use the new component in ee/app/assets/javascripts/ci/merge_trains/merge_trains_app.vue
4. Add basic test coverage for the component.