Turns out that Microsoft Windows, Chrome OS, Safari and others all
have their own little precious way of using the traditionnal favicon,
which are all pretty much incompatible with each other. Fixing this
requires regenerating not one, but *seven* different images, and *two*
metadata files that are somehow magically read by different
browsers/environments.

This is a shame and an embarrassment, but I prefer to have pretty
icons for users than try to fight such a useless battle.

This requires changes to the theme as well, which should hopefully
happen at the same time as this change.

Mostly cargo-culted from https://realfavicongenerator.net/

Original image should be found in 2013/10/14/prague/IMG_2576.CR2 in my
Photos archive.