Show error message when pipelines disabled and Pipeline must succeed

This is the first step towards fixing the Epic #12686 (closed) to show accurate messages when MR button is disabled. This specific issue should not focus on the problem as a whole, but a very specific instance where the settings are simply invalid and we give no help to users.

• Pipelines are disabled
• MR pipeline must succeed is turned on
• No external CI is setup

This correspond to option C in the EPIC:

C. Pipelines are disabled and there is no pipeline information available for the commit (pipelines were not enabled or there was no ci configuration when the commit was created):

• Settings -> General -> Merge Requests -> Merge checks -> "Pipelines must succeed" is enabled
• Settings -> General -> Visibility, project features, permissions -> "Pipelines" is disabled
• Pipeline information is generally not shown anymore except in merge requests where it is required to succeed.
• Pipeline section of merge request widget states: No pipeline was run for this commit.
• The merge section of the merge request widget states: Pipelines must succeed for merge requests to be eligible to merge. Please enable pipelines for this project to continue. For more information, see the documentation.

Screenshots

Before

After

app/assets/javascripts/vue_merge_request_widget/components/states/ready_to_merge.vue

@@ -58,7 +58,7 @@ export default {
     status() {
       const { pipeline, isPipelineFailed, hasCI, ciStatus } = this.mr;
 
-      if (hasCI && !ciStatus) {
+      if ((hasCI && !ciStatus) || this.hasPipelineMustSucceedConflict) {
         return 'failed';
       } else if (this.isAutoMergeAvailable) {
         return 'pending';
@@ -83,7 +83,7 @@ export default {
         this.status === 'failed' ||
         !this.commitMessage.length ||
         !this.mr.isMergeAllowed ||
-        !this.mr.isApproved
+        this.mr.preventMerge
       ) {
         return 'warning';
       }