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

@@ -326,10 +326,7 @@ export default {
               </li>
             </ul>
           </span>
-          <div
-            class="media-body-wrap"
-            :class="{ 'space-children': !hasPipelineMustSucceedConflict }"
-          >
+          <div class="media-body-wrap space-children">
             <template v-if="shouldShowMergeControls">
               <label v-if="mr.canRemoveSourceBranch">
                 <input