MR notes improvements

Description

There's an inconvenience in the code review UX.

When a discussion on a merge request is outdated because the requester fixed it, I want to see how the code is changed.
However, there is no direct link to the changed lines.
When opening a new tab to see all changes, code is chunky.

Proposal

UI should show the discussed code and the changed code together.

Links / references

https://gitlab.com/gitlab-org/gitlab-ee/issues/580

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