Implement a word-diff mode for merge requests

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Belongs to &1373 epic.

Prerequisites

1. Word-diff parser - !55105 (merged)
2. Word-diff support on gitaly side - #324805 (closed)

Proposal

We want to display a git diff --word-diff response on UI. User will have an option to choose Word-diff mode on merge request screen.

Technical implementation

User sends an extra attribute to enable word-diff mode to the backend controller.

GitLab ruby application will pass this attribute to Gitaly via Gitaly::CommitDiffRequest.

The response will be parsed by Gitlab::WordDiff::Parser and returned back to the frontend.

Screenshots

The expected result approximately looks like this: