French translation - Traduction Françes

Started french translation

- copied `_posts/en` to `_posts/fr`
- adjusted all categories and paths in `_posts/fr/*`
- copied global strings in `_config.yml`
- pulled in existing translations by @PiRK from [his fork](https://github.com/PiRK/learnosm/tree/9bcec78fd37babd7517e543ed2781649f87f24fc/_posts/fr)

Todo:

- All translations in `_config.yml`
- Remaining translations in `_posts/fr`
- Add "Français" link to language switcher
- Full review
- Merge 

**This is not ready to be merged yet**