Add pipeline name to Slack pipeline notifications

What does this MR do and why?

Describe in detail what your merge request does and why.

Pipelines can now have custom names set by a new name attribute in .gitlab-ci.yml. This MR adds the name to pipeline notifications sent to Slack/Mattermost. It will help users identify which pipeline generated the notification without having to open the pipeline.

All changes related to the new pipeline name feature are behind the pipeline_name feature flag, including this MR.

#372538 (closed)

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	With pipeline name	With long pipeline name	Without pipeline name or feature flag disabled
Successful pipeline
Failed pipeline

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