web: diff lines, files and review threads can be linked to !221

The first piece of #118. Diff rows carried no id, so a review comment or a line under discussion could not be pointed at, and a file page's #L<n> anchors had no highlight.

Each diff file is #f<n>, each row #f<n>-n<line> (or -o<line> for a deleted line), each review thread #thread-<id>, and a targeted row, file line or thread is highlighted with the orange mark: tr:target on diffs, .line:has(a:target) on files, .thread:target on threads. No JavaScript, as before.

Stacked on !220; no code dependency.

Ref #118