Add merge train position message (CE)

This commit adds an informational message under the pipeline block in
the merge request widget that displays the current position of the merge
request on the merge train.

app/assets/javascripts/vue_merge_request_widget/components/mr_widget_pipeline_container.vue

 <script>
+import _ from 'underscore';
 import Deployment from './deployment.vue';
 import MrWidgetContainer from './mr_widget_container.vue';
 import MrWidgetPipeline from './mr_widget_pipeline.vue';
@@ -17,6 +18,8 @@ export default {
     Deployment,
     MrWidgetContainer,
     MrWidgetPipeline,
+    MergeTrainInfo: () =>
+      import('ee_component/vue_merge_request_widget/components/merge_train_info.vue'),
   },
   props: {
     mr: {
@@ -58,6 +61,9 @@ export default {
     showVisualReviewAppLink() {
       return Boolean(this.mr.visualReviewFF && this.mr.visualReviewAppAvailable);
     },
+    showMergeTrainInfo() {
+      return _.isNumber(this.mr.mergeTrainIndex);
+    },
   },
 };
 </script>
@@ -83,6 +89,11 @@ export default {
           :visual-review-app-meta="visualReviewAppMeta"
         />
       </div>
+      <merge-train-info
+        v-if="showMergeTrainInfo"
+        class="mr-widget-extension"
+        :merge-train-index="mr.mergeTrainIndex"
+      />
     </template>
   </mr-widget-container>
 </template>

app/assets/stylesheets/pages/merge_requests.scss

@@ -904,7 +904,8 @@
   margin-right: -5px;
 }
-.deploy-heading {
+.deploy-heading,
+.merge-train-info {
   @include media-breakpoint-up(md) {
     padding: $gl-padding-8 $gl-padding;
   }