Add Libertinus Serif

_(Originally reported at https://github.com/elementary/seed-metapackages/pull/5)_

[Linux Libertine](http://www.linuxlibertine.org/) ([Wikipedia](https://en.wikipedia.org/wiki/Linux_Libertine)) provides an independently-designed, high-quality serif font. (Update: the original project has become stale, but there's an actively maintained fork, [Libertinus](https://github.com/alerque/libertinus).)

The design is independent, unlike the (already included) Liberation family of fonts, which offers metrically identical alternatives to the widespread fonts Times New Roman, Arial and Courier New.

I was actually quite surprised that this font wasn't included by default in a popular Linux distro. I'm now worried that I can't rely on it being present (e.g. for stylesheets like [downstyle](https://github.com/waldyrious/downstyle/issues/15)). Are you aware of any explicit or implicit standards/recommendations for fonts that are included in distros, @codygarver? Also, was elementary's current selection organically grown, or are there guidelines for what should or not be included?