Fix border display of table header cells

[waldyrious]

Originally submitted at https://github.com/jgm/djot/pull/177.

The stylesheet had a rule that was probably a typo, since it adds a border to the entire rows (<tr>), which is redundant with the border added to the cells (<td>); and it fails to add a border to <th> elements. So the following table:

| test | foo |
|------|-----|
| bar  | baz |

currently looks like this:

With this change (in particular, 3f6ce3c), it will look like this:

Note that I also added some cell padding (5927408).

[waldyrious]

@jgm I've moved the changes from https://github.com/jgm/djot/pull/177 to here. This time I made sure to make separate commits for each semantically atomic change, so I'd appreciate it if you merged without squashing the commits. Cheers!

[jgm]

Thanks!