Implement support of comparing diff versions

What does this MR do and why?

When comparing version to latest, we need to support finding a MergeRequestDiff by diff_id from params. We also need to support comparing versions (by diff_id and start_sha params).

To support that, we modify Gitlab::MergeRequests::DiffVersion to do that depending on the params set.

This is behind rapid_diffs_on_mr_show feature flag.

References

#466149 (closed)

How to set up and validate locally

1. Select a diff version in MR page.
2. Copy the URL of that page and append rapid_diffs=true param.
3. Compare diffs and they should match.
4. Compare 2 diff versions in MR page.
5. Copy the URL of that page and append rapid_diffs=true param.
6. Compare diffs and they should match.

MR acceptance checklist

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Related to #466149 (closed)