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

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

It wasn't ever necessary. Still allow configuring this path in the GUI in case of different paths to static files for scratch-blocks.