API JS i18n files with short locale names, generated by build task

The files are uploaded to transifex.com now, which means we needed
to consolidate the file names (= locales). Since Transifex doesn't allow
arbitrary JavaScript structures, we're using an intermediary JSON
format which gets transformed into the JS used by the framework.

See https://github.com/silverstripe/silverstripe-buildtools/commit/353359715a857e6480f2063ff622ea41f4aba765 for generator logic.

Fixes https://github.com/silverstripe/silverstripe-framework/issues/2499