Add skeleton loader to the pipeline tab on merge request page

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Release notes

Problem to solve

The pipeline list takes time to load. The cause is being investigated in #341462 (closed), but it would also be useful to replace the spinner with a skeleton loader for improving perceived performance.

Proposal

provide an SVG for the skeleton loader for Merge request>> Pipelines page.

Intended users

Feature Usage Metrics

Related to #341462 (closed).

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