Add word-diff option to merge requests

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Problem to solve

When working with markdown (or any type of prose/text in general), the "classic" git-diff (intended for code) is of limited use.

Proposal

With the logic changes introduced in #16950 (closed) and after Gitaly merges support for word-diff, we should add a user control to enable word-diff mode in merge requests.

Further info

• Word diff parser !55105 (merged)
• Gitlab::Diff::Highlight refactor #323763 (closed)
• Word-diff Gitaly support gitaly!3086 (merged)