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

@@ -219,6 +219,10 @@ def cancelable?
       statuses.cancelable.any?
     end
 
+    def auto_canceled?
+      canceled? && auto_canceled_by_id?
+    end
+
     def cancel_running
       Gitlab::OptimisticLocking.retry_lock(
         statuses.cancelable) do |cancelable|