Extract all translatable strings from front-end code

[jnm]

Related to kobotoolbox/form-builder-translations#3, we want to automatically extract strings from the UI code without a manual procedure that uses a desktop application. This will cut down on developer busywork and the long delay between application changes and updated translations.

1. babel-loader loads ES6 sources and coffee-loader loads the CoffeeScript sources (i.e. form builder);
2. We avoid calls to t() being turned into things like _utils__WEBPACK_IMPORTED_MODULE_8__["t"]) by not importing t but using webpack.ProvidePlugin instead;
3. We use https://github.com/grassator/webpack-extract-translation-keys, which hooks into webpack's JavascriptParser to extract all static strings passed to t();
4. We process the resulting file into something useful for upload to Transifex (TBD; let's address as part of Explore Transifex GitHub integration form-builder-translations#3).

In the future, when all code is ES6 or beyond, we may be better served by https://github.com/ttag-org/ttag.