Unified translation format and web-based translations

The idea here is that we're going to start managing translations through Transifex: https://www.transifex.com/projects/p/pioneer/

So, we have the following changes:

* Converts all translation strings (core + Lua) to the [Chrome i18n .json](https://developer.chrome.com/extensions/i18n.html) format.
* Removes language fallbacks, since Transifex will ensure that all strings are defined.
* One language resource per module, and one language per file. Much better for modding.
* No more concept of arbitrary per-language flavours. All strings in a resource have to exist for all languages.

I've started some docs to explain all this, but I've got to update the existing translation docs on the wiki.

Next step after this is to have an automated process to pull new translations from Transifex and commit them each day. Its more of a build system concern though, it can come after.