Improve validator

[dbrgn]

Current HTML code: https://github.com/SpaceApi/website/blob/master/templates/validator.html

Validator is here: https://github.com/SpaceApi/website/blob/ba32846f47706d64938296ee03cbd6cf55823f09/webpack/js/validator.js

If someone wants to work on this, it's fine to completely throw away the current code and rewrite it!

A solution with as few dependencies as possible is welcome, to make maintenance easier.