User who is not allowed to trigger manual actions should not be
allowed to reprocess / retrigger / retry these actions.

Turn ExpirePipelineCacheService into Worker so it can fetch all the
merge requests for which the pipeline runs or did run against.

Also poll for pipeline changes on:
 - Pipeline table on commit page
 - Pipeline table on merge request page
 - Pipeline table on new merge request page

Enable caching in the Etag::Middleware and when a pipeline changes
status, expire the cache for the project pipelines path.

Not only tracking auto-cancelling in pipelines,
we'll also track this in jobs because pipelines
could be retried and the information would get lost
when this happened. Also erase auto-cancelling
relation for pipelines when they're retried.

fix changelog MR reference

add non-HEAD builds finder and add `created` pipelines to scope

add spec for auto-cancel non-HEAD pipelines and refactor create_pipeline_service_spec

more refactoring for spec

adds option for auto-cancel into CI/CD settings

fix spec to new configuration

fix rubocop

fix schema.rb

fix schema.rb

replace Gitlab 9.0 with 9.1 in doc

change wording on pipeline settings

added auto_canceled_by field as identifier of autocancel subject

remove unnecessary index

replace service with retry_lock

replace auto_cancel_pending_pipelines boolean setting with integer (and enum in model)

fix schema.rb

fix schema.rb

remove projekt attribute and clean up spec

clean up spec withcouple of shared examples

added spec for "It does not cancel current pipeline" scenario

add some specs to auto-cancel

add spec for another branch pipelines

Do not skip manual actions even if actions are optional.