Add "compare with latest version" link to "X approved" system note
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I might not be obvious which revision of the code a user might have approved with the following system note:

### Proposal
Append a link to "compare with latest version" to that message:
> `@user` approved this merge request 1 day ago. [compare with latest version](#)
That link will take you to the Diff tab with the approved version vs the latest version selected.
issue