Fixes #4413

There's a fair amount of discussion in the issue but the main points are:

• Visiting a URL with the zh_CN and zh_TW locales does not translate the content.
• Babel / flask-babel expects folders named zh_Hans_CN and zh_Hant_TW.
• We want to fix this, but keep old URLs like http://localhost:5000/zh_CN working (new one will be http://localhost:5000/zh_Hans_CN)
• Transifex expects zh_CN and zh_TW

So this PR does:

1. Rename the locale folders to what Babel expects
2. Sets up redirect routes for the old legacy locale names (eg http://localhost:5000/zh_CN/dataset ->
   http://localhost:5000/zh_Hans_CN/dataset)
3. Adds an alias to the Transifex config file to be able to push/pull normally