Make pipeline failure Slack notifications prettier and more informative

What does this MR do?

Improves Slack pipeline status notifications. Here are examples of the cases this MR handles:

Pipeline Failure (1 failed job)

Pipeline Failure (13 failed jobs)

Note: Since taking this screenshot, I've lowered the maximum threshold to 10, so the and X more text will actually show when at least 11 jobs fail.

Pipeline Failure (2 failed stages)

Pipeline Failure (YAML error)

Pipeline success

Pipeline success (with warnings)

Some additional edge cases:

Really long job names

(Job names are currently not truncated in any way.)

Really long stage names

(Stage names are currently not truncated in any way.)

Really long branch name

(Branch names are currently not truncated in any way.)

Really long commit messages

Only the first line of the commit message is shown. This message is truncated if it exceeds 100 characters.

Current state

Just for reference, here is what a pipeline failure notification currently looks like:

EE port

https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/12635

It appears this port is no longer needed.

Feature flag

This MR hides its changes behind a feature flag named fancy_pipeline_slack_notifications. The feature flag is enabled by default.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Closes #32495 (closed)