web: range-diff page for merge request revisions #269

The web has no range-diff view; the MR page sends reviewers to the CLI.

MR 471 reads "2 revisions pushed. What changed between the last two: gitbay mr range-diff krz/gitbay 471" (mr.html:138). A rebase stales approvals, so a reviewer re-approving on the web needs to see what the force-push changed. The CLI and iOS already have this view.

• /{owner}/{repo}/mrs/{n}/range-diff?from=&to=, dispatching mr range-diff, rendered as text like the iOS view.
• A "compare to previous" link on each revision row.
• Update Parity.