Generate `locale/messages.json` from available languages in `translations`.  If there are no translations for a given language (from `languages.json`) it just skips it, react-intl will use the default english strings.

Just using English, French and Spanish for testing right now.

Making the script easier to reuse later.

Using babel-plugin-react intl to extract messages from sources that use react-intl. It creates one file per component, that’s not ideal for translators.

Transifex supports several different json formats. We picked the chrome i18n json format because it includes the ‘description’ and displays that to translators as instructions in the transifex UI.