Code block formatting fixes

Before, the line number column was sized incorrectly on Chromium, overly smushed on Firefox, and nonexistent on GNOME Web. In addition, content that overflows past the edge can be properly scrolled now.

Tested in Chrome 98, Firefox 96, and GNOME Web 41.

Signed-off-by: Ryan Gonzalez ryan.gonzalez@collabora.com

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Some quick example images, taken in Chrome Devtools (screen size = Pixel 5, both images downscaled to 512px width because the original screenshots were far too massive in the MR description):

Before:

After:

The giant code block column looked even worse better on desktop:

Once again, I will only marginally apologize for the stripped trailing whitespace, because it's evil and GitLab can hide whitespace changes when viewing diffs anyway.