Recent version of django moved zh-tw and zh-cn locales to zh-hant and zh-hans. There is some code for backwards-compatibility, but that will go away and we'll need to handle that.

I had started doing that in mozilla/addons-server#5811 but it requires more thought so I reverted that commit and moved the discussion here. One of the issues is that if we change the locales like this, we need to add redirects from the old locales and migrate data in translated fields.

This blocks #4292 (See #4515)