web/style.less: Separate out .table-bordered from .table

The only instance of .table in the web source is the download page for the
Git repos. I did not change that page because an unbordered table there is
prettier IMO.

I retained the .table-bordered style in case anyone want to use it in the
future.

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Reviewed-by: Clément Bœsch <u@pkh.me>