Move localization to client

[fheinfling]

The issue has been raised after a new locale has been added in #8. When it comes to localization we're currently using a server and client-side approach.

Currently only remote (push) notifications are localized on server-side. E.g. in https://github.com/janoodleFTW/timy-messenger/blob/440fda86c339c50f3bec807eb9a4dca73fe806ca/firebase/functions/localize-util.js#L5

We should consider replacing the server-side approach with localizing solely on our clients.

FYI: Previously also system messages were localized in our backend but have been moved back to our clients using the approach described in https://github.com/janoodleFTW/timy-messenger/pull/8#issuecomment-539894286