Frontend : add `<MR version>` switch to merge request's Commits tab and `Code` -> `Download` links
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## Summary
On the merge requests page, at the `Changes` tab there is the `Compare <branch> and <MR version>` switch.
At the `Commits` tab, there is _no_ such (`<MR version>`) switch.
At the `Code` -> `Downloads` -> `Patches`/`Diff` links, there is _no_ such (`<MR version>`) switch.
### Why this matters and how we measure
1. Inconsistency.
2. I can't see the previous versions' commits/patches/diffs.
## Proposal
At the `Commits` tab, add the `<MR version>` switch showing the appropriate MR version's commits.
At the `Code` -> `Downloads` -> `Patches`/`Diff` links, add the `<MR version>` switch letting download the appropriate MR version's patches/diff.
Thanks!
issue