Dynamic import didn’t do what we wanted, so just added `npm run i18n:msgs` to the start command. This is just temporary. Other changes as requested:
* exports in one place
* loc=> locale

Also fixed use of require for localeData to use dynamic import (this one works), so the babelrc changes are needed, otherwise import as a function is not recognized.

Using react-intl-redux package.

This just sets up the default English. Language switching in another PR. Requires locale/messages.json to exist, `{}` is ok.

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.

Generating the localization source to update the translation system (transifex) is really separate from building. When building we want to be using the current state of translations from transifex, and the english default strings are not needed.

Uses the babel-cli to run the babel-plugin-react-intl separately from  webpack.

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.

Costumes and Sound tab strings.

Closes #429