Replace the default avatar with a SVG version.

This is both lighter, looks better when rendered at arbitrary
resolutions (as there is no scaling being done), and actually editable
for different deployments.

This image has been hand-drawn using the PNG as a background.

css/images/user.svg

@@ -0,0 +1,6 @@
+<?xml version="1.0"?>
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 128 128">
+ <rect width="128" height="128" fill="#555"/>
+ <circle cx="64" cy="41" r="24" fill="#fff"/>
+ <path d="m28.5 112 v-12 c0-12 10-24 24-24 h23 c14 0 24 12 24 24 v12" fill="#fff"/>
+</svg>