Set up a system for contributing translations

[takluyver]

There are sites like Transifex which provide an interface for people to contribute to translating a project. We should set one up so there's an easier way for people to add translations.

[jtpio]

Closing as Notebook 7 uses the same localization infrastructure as JupyterLab via Crowdin: https://github.com/jupyterlab/jupyterlab-translate

Thanks!