I took our website's SVG favicon and plopped it on a round
white rectangle in Affinity Designer. The I exported it as an SVG and
wrote a script that uses imagemagick to convert to the various sizes and
formats we need.
Closes#2307
Might fix#1181, although not for the reasons I initially
thought (because the URLs are resolved from the manifest path, not the
path of the current page). This should ensure that the URLs used by the
manifest are always correct regardless of the manifest's path.
