build: Remove reactifex dependency.

This repo only relies on that deprecated library for one script and
since this repo is also on the road to deprecation, for now I've just
copied the one script we needed from that repo to here so that we can
drop that dependency and archive that repo.

Author: feanil

Makefile

@@ -2,7 +2,7 @@ UNAME := $(shell uname)
 
 i18n = ./src/i18n
 transifex_input = $(i18n)/transifex_input.json
-transifex_utils = ./node_modules/.bin/edx_reactifex
+transifex_utils = node ./scripts/concat_translations.js
 
 # This directory must match .babelrc .
 transifex_temp = ./temp/babel-plugin-react-intl