Added merge checks component to mr_widget_options

What does this MR do and why?

This adds the checks component into the widget app so that it renders to users. Also added realtime to the merge checks component by using an existing subscription.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

• Enable merge_blocked_component feature flag
• Visit a merge request
• See Merge blocked... component in the merge widget

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

app/assets/javascripts/vue_merge_request_widget/stores/mr_widget_store.js

@@ -196,7 +196,9 @@ export default class MergeRequestStore {
     }
 
     this.commitsCount = mergeRequest.commitCount;
-    this.branchMissing = !mergeRequest.sourceBranchExists || !mergeRequest.targetBranchExists;
+    this.branchMissing =
+      mergeRequest.detailedMergeStatus !== 'NOT_OPEN' &&
+      (!mergeRequest.sourceBranchExists || !mergeRequest.targetBranchExists);
     this.hasConflicts = mergeRequest.conflicts;
     this.hasMergeableDiscussionsState = mergeRequest.mergeableDiscussionsState === false;
     this.mergeError = mergeRequest.mergeError;