This change updates the text of the pipeline widget that appears on the
merge request page. The text has been made more consistent between
different types of pipelines; this makes the front-end implementation
simpler and more maintainable.  In addition, the type of pipeline is
(i.e. regular pipeline, merge request pipeline, detached pipeline)
included in the text, making this type more obvious to the end user.

Some information has been removed from the widget as part of this
change; however, any information that was removed already appears
elsewhere on the merge request page.

Detect if pipeline runs for a GitHub pull request

When using a mirror for CI/CD only we register a pull_request
webhook. When a pull_request webhook is received, if the
source branch SHA matches the actual head of the branch in the
repository we create immediately a new pipeline for the
external pull request. Otherwise we store the
pull request info for when the push webhook is received.

When using "only/except: external_pull_requests" we can detect
if the pipeline has a open pull request on GitHub and create or
not the job based on that.

Since it is not possible to dynamically detect if a job is automatically
cancellable or not, a this new attribute is necessary. Moreover, it let
the maintainer of the repo to adjust the behaviour of the auto cancellation
feature to match exactly what he needs.

This commit adds pipeline.type key to PipelineEntity.
This key will be used in MR widget in the next iteration.

This is a port from EE changes where
we introduce a new limit for Plan model.

https://dev.gitlab.org/gitlab/gitlab-ee/merge_requests/1182

Ports changes from
https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/12343