Add favicon to web frontend

Add a favicon to the web frontend.

This is especially useful for those
who pin the Warrior tab, or make it into a standalone app (via
Chrome's "Create application shortcuts" for example).

warrior-small.png is alard's [warrior.png from universal-tracker](https://github.com/ArchiveTeam/universal-tracker/blob/7fa95df3af77e831ba3f570e54c35f19c9bb5b69/public/img/warrior.png),
1d957acdb185d41cc93b9a6441447497477a7eb8