Use localisation service to make translating croodle easier

[jelhan]

Adding new locales and maintaining existing ones is currently more difficult and error-prune as it should be especially for contributors that aren't used to software development in general and GitHub workflow in special. Also keeping track of progress is difficult as there isn't any statistics about how many texts are missing translations for a specific language. Additionally several files needs to be changed as soon as a new translation key is added.

All issues mentioned above could be addressed by using a localisation service that decouples translating the application from software development. A quick research brought up lokalise.co, Transifex and Weblate. I'm tending towards Transifex cause that one is also used by Discourse, which is a large open source web application build with Ember. So we might could profit from their experiences.