Fix style-lint for `ee/app/assets/stylesheets/pages/pipelines.scss`

What does this MR do?

This MR fixes the stylelint errors for pipelines.scss. The three commits are grouped by warning, as listed in #10950 (closed).

change	Before	After
issue 1: .has-linked-pipelines big graph
issue 1: .has-linked-pipelines mini graph, prefix
issue 1: .has-linked-pipelines mini graph, postfix
issue 2: replace icon nesting with utility class
issue 3: set spacing with utility classes, prefix
issue 3: set spacing with utility classes, postfix

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 #10950 (closed)