Language as a Site Groundwork

This lays the groundwork for language as a site. The site is switched to match the user's current language preference if a matching site is found. This works in the admin using the existing the language switcher. The site switcher has be removed since it is somewhat redundant. New language sites can be created with a new management command `build_language_sites` which copy the currently CMS content from the default English version of the site i.e
```
python manage.py build_language_sites zh-cn
```

Remaining work for future tickets:

- Frontend language/site switcher
- Linking back to the English content in the admin
- Tracking updates to the English content for the translators