Retire the grunt-based build

Right now, GitGitGadget's HTML pages are built using `grunt`, and the
result is then committed and uploaded to GitHub Pages.

However, a much more natural development experience can be achieved by
using Hugo (https://gohugo.io/) instead.

In preparation for migrating to Hugo, let's drop all `grunt`-based
stuff.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho