Add CSS rule to prevent excess left margin

What does this MR do?

This fixes the extra left margin that shows in the pre HTML elements by adding a gl-white-space-pre-line css class

The above class will be added to gitlab-ui in the following MR: gitlab-ui!1846 (merged). The fix should kick in as soon as the gitlab-ui version is updated in this repository

#214587 (closed)

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Before	After

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content