Minor cosmetic changes to the source viewer

What does this MR do and why?

As we're getting ready to roll out syntax highlighting with Highligh.js we need to ensure the source viewer is looking as expected.

This fixes some minor cosmetic issues in the source viewer.

⚠ note: changes in this MR are behind the highlight_js feature flag.

Screenshots or screen recordings

before	after

How to set up and validate locally

1. Enable the :highlight_js feature flag
2. Open a source file in the repository files browser

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.