client (js) side of Czech language localization (PR #299)

Czech localization (only client now, will work on server translations).

GitHub

You’ve signed the CLA, kubabrecka. Thank you! This pull request is ready for review.

Looks good - to make it active you’ll have to add a cs.js.erb to locales though, but I’m more than happy to accept work in progress localizations :slight_smile: