Show indicator to Pipelines for merge train

What does this MR do?

Updates the text of the pipeline widget on a merge request's page.

This new text simplifies the current implementation (which has an unwieldy number of v-if in the Vue template) and also better indicates the pipeline's purpose.

Current implementation

(The current implementation has many possible permutations; only one scenario is shown in the screenshot above.)

UX proposal:

Note: the conditions will be evaluated in the order specified below.

Pipeline type	Pipeline texts in MR	Evaluation in FE
Pipelines for merge train	Merge train pipeline # 123 pending for adcd1234	pipeline.flags.merge_train_pipeline === true
Pipelines for merged results	Merged result pipeline # 123 pending for adcd1234	pipeline.flags.merge_request_pipeline === true
Pipelines for merge requests (detached)	Detached merge request pipeline # 123 pending for adcd1234	pipeline.flags.detached_merge_request_pipeline === true
Branch pipelines	Pipeline # 123 pending for adcd1234 on feature-branch	pipeline.ref.branch === true
Tag pipelines	Pipeline # 123 pending for adcd12341	pipeline.ref.tag === true

¹It is not currently possible to create a merge request for a tag, so this scenario is N/A at the moment. However, this feature has been proposed, so this merge request attempts to handle this possibility gracefully.

Screenshots

Merge train pipeline:

Merged result pipeline:

Detached merge request pipeline:

Branch pipeline:

Tag pipeline: N/A (tag pipelines are not yet implemented by GitLab)

Related issues

• https://gitlab.com/gitlab-org/gitlab-ee/issues/11222

This MR has a CE backport

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/32025

ee/spec/factories/merge_requests.rb

@@ -24,6 +24,15 @@
       end
     end
 
+    trait :with_merge_train_pipeline do
+      with_merge_request_pipeline
+
+      after(:create) do |merge_request, evaluator|
+        merge_request.pipelines_for_merge_request.last
+          .update(ref: merge_request.train_ref_path)
+      end
+    end
+
     trait :add_to_merge_train_when_pipeline_succeeds do
       auto_merge_enabled true
       auto_merge_strategy AutoMergeService::STRATEGY_ADD_TO_MERGE_TRAIN_WHEN_PIPELINE_SUCCEEDS