Display change that made thread outdated in-line

As of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11584, any time new commits are pushed into an MR that directly touch lines that had diff threads on them, these threads are marked as outdated and get a system note with a link to the comparison between the two MR versions, and the exact line in that diff that represents that change.

This is useful for reviewers who may want to see how, exactly, their concerns ended up being addressed, before they mark the thread as resolved, or to verify that a thread that was resolved by the MR author was indeed addressed as expected.

However, the MR diffs page can be quite slow to load, especially when comparing two different versions (because the comparison isn't cached), and especially when the push involved a rebase or merged commit, which can result in changes being included in the diff that aren't actually relevant. (See also &340).

I would love it if the change diff was displayed in-line, as part of the system note, potentially behind a "Toggle diff" switch, as below:

It could just be a simple static diff with a few lines of context; if I want to see more context, or comment on the change, I can follow the "version X of the diff" link.

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