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.

Added language-selector component and container. Replaced lib/intl.jsx with reducers/intl.js and added updateIntl to handle language changes.

Moved languages.json into src

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.

Keep localization in the container level. Ideally we should not be passing message descriptors.

Costumes and Sound tab strings.

In order to comply to WCAG 2.0[1], buttons must have discernible text that
clearly describe the purpose, so that it can be used by screen readers.

[1] https://www.w3.org/TR/2016/NOTE-WCAG20-TECHS-20161007/H65