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

timy-messenger/firebase/functions/localize-util.js

Line 5 in 440fda8

* Building notification title for RSVP update for event author.

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 #8 (comment)