Close new game modal after clicking outside

[randompearl-stack]

I don't think this is going to work...

finishes #1356

[gbtami]

This doesn't work indeed, but I managed to fix it via https://github.com/gbtami/pychess-variants/commit/d6fd5872c8ae620b70670453c9cbdc80c6beeb69 after reading https://www.w3schools.com/howto/howto_css_modals.asp