In commit d8b10296 ("footer: use h6 instead of strong"), I opted for a
bit more semantical HTML, but it seems that dropping the strong-tag here
had an unexpected side-effect; the boldness might have just been a bit
less on my system, but it seems on other systems it's a *lot* less bold.
In fact, not bold at all.

It turns out, this depends on the fonts installed on the system, and
Bootstrap uses a default font-weight of 500 for headings. This looks
fine most places, but in the footer here, we *really* want the headings
to stand out from the rest.

So let's add back some boldness. By doing this in the CSS instead, we
get to keep the semanticness of the HTML without having to sacrifice the
look. The value of 600 was picked by a bit of trial-and-error; both
systems I have where this doesn't render well as-is is fixed at 550 and
above, but let's bump it a bit more to be sure.