WPML Compatibility

LifterLMS doesn't work well with [WPML](https://wpml.org/)

This issue collects various compatibility issues with the plugin

- Translating a Course does not bring along children (sections, lessons, etc...). Probably because WPML doesn't have any knowledge of the course's children relationships (because why would they). My guess is that "translating" duplicates the post and stores metadata for the locale. We'd need to hook into this translation action and run a clone on the course to setup translations for the children

- Syllabus widget does not work on translated versions. This makes me question the above assumptions. I don't know.

WPML Resources:
+ https://wpml.org/documentation/theme-compatibility/go-global-program/#contact
+ https://wpml.org/faq/newly-activated-plugin-not-compatible-wpml/
+ https://wpml.org/documentation/support/achieving-wpml-compatibility-for-your-themes-and-plugins/

For WPML support we need to document the following which will require translations:

- [x] ~Custom Post Types~ (#772)
- [x] ~Custom Taxonomies~ (#773)
- [ ] Custom Fields
- [ ] Options