Added 404.html to handle reloading on different endpoints.

[b-smiley]

When I first used this template it caused a 404 error because react is single page and Github is looking for .html files.

NOTE: This code came directly from spa-github-pages so we may need to reference this repo better.

This is my first public commit ever! Sorry if its wrong.

[b-smiley]

Closes #55