Wrapping was initially added in https://github.com/go-gitea/gitea/pull/2789
but it is currently disabled because the rule that applies wrapping to
the wrap class did not have enough CSS specificity.
Resolved the issue by using a general selector that matches all code
boxes. The previous wrap class was removed because it had only been
inconsistenly applied to various code boxes and because I found no easy
way to add classes to code boxes rendered in Markdown.
Also removed a seemingly useless :before on code view and added padding
to restore the spacing.
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