Show pipeline status when linking to branch or pipeline from markdown

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What

Add a success/pending/failed indicator when a branch URL or pipeline URL is included in issue descriptions or comments.

Why

Allow us to mention branches in issues to raise visibility about their status. Useful in places like a release checklist which might ask to ensure the pipeline is green for a given branch.

Came up in slack discussion (https://gitlab.slack.com/archives/C0GQHHPGW/p1497981298919934) after an RC merge failed without us noticing

Why not

It might add visual clutter and effect performance, so these should be considered first.