Add auto-cancel for pending pipelines on branch, if they are not HEAD

What does this MR do?

Decrease entropy in Gitlab universe 😸

First iteration for #8998 (closed)

• Adds option "Auto-cancel pending pipelines" in "CI/CD Pipelines" project settings tab: when checkbox checked, all pending non-HEAD pipelines will be canceled by system, automatizing scenario:

  1. "User creates pipeline for branch" / "User pushes commit to gitlab"
  2. User goes to pipelines index page and cancel unnecessary (only pending) pipelines.

• Adds target: '_blank' to doc links in "CI/CD Pipelines" project settings tab

• Refactor spec/services/ci/create_pipeline_service_spec.rb, extracting execute_service method

UI:

• Pipeline list:
  • Tooltip on canceled status badge
• MR widget/commit widget:
  • Pipeline #12345678 was auto-canceled.. etc
  • Hovering over the words auto-canceled will bring the same tooltips
• Pipeline page:
  • Tooltip on canceled status badge
• Pipeline graph job node:
  • has improved tooltip with auto-canceled (not anything else)
• Mini pipeline graph stage and job status icons
  • has improved tooltip with auto-canceled (not anything else)
• Job list
  • Tooltip on canceled status badge
• Job list in pipeline view
  • Tooltip on canceled status badge
• Job view
  • Tooltip on canceled status badge

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Many reasons are discussed here #8998 (closed)

Screenshots (if relevant)

Option in CI/CD Pipelines Project settings tab:

Pipeline list:

Pipeline page:

Job list:

Job view:

Job list in pipeline view:

Setting:

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

#8998 (closed)

app/models/ci/pipeline.rb

@@ -70,6 +70,10 @@ class Pipeline < ActiveRecord::Base
         pipeline.update_duration
       end
 
+      before_transition :canceled => any - [:canceled] do |pipeline|
+        pipeline.auto_canceled_by = nil
+      end
+
       after_transition [:created, :pending] => :running do |pipeline|
         pipeline.run_after_commit { PipelineMetricsWorker.perform_async(id) }
       end
@@ -95,10 +99,6 @@ class Pipeline < ActiveRecord::Base
           PipelineNotificationWorker.perform_async(pipeline.id)
         end
       end
-
-      after_transition :canceled => any - [:canceled] do |pipeline|
-        pipeline.update(auto_canceled_by: nil)
-      end
     end
 
     # ref can't be HEAD or SHA, can only be branch/tag name