Discussions Vue refactor for Commits and Snippets page

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Commits and Snippets discussions are still using legacy notes.js and bunch of Haml templates. We should refactor those pages to use our Vue discussion components.

Some of the benefits that I can think of,

• We will have a unified design in discussion across GitLab
• We will delete long-standing legacy notes.js and a lot of Haml templates
• We will only one code path for discussions

We should also rewrite Commit diff with Vue since we already have diff file components ready. We have another issue for this https://gitlab.com/gitlab-org/gitlab-ce/issues/49620