R1:98fe8fae4a90

Use `<td class="n" data-n="3">` instead of `<th>3</th>` for line numbers

Summary:
Ref T13161. Ref T12822. See PHI870. Long ago, the web was simple. You could leave your doors unlocked, you knew all your neighbors, crime hadn't been invented yet, and `<th>3</th>` was a perfectly fine way to render a line number cell containing the number "3".

But times have changed!

- In PHI870, this isn't good for screenreaders. We can't do much about this, so switch to `<td>`.
- In D19349 / T13105 and elsewhere…