Add Internationalization support

[it-vegard]

The admin tool needs to support multiple languages - Norwegian and English by default, but hopefully others can add support for other languages eventually.

Not sure on how to solve this, though. Enonic XP has i18n support, but since the admin tool is a React component, it is probably best to use a React/Redux library, like React Intl.

The goal must be to not have to change the markup substantiately, have no hard-coded texts outside of the phrases-files and to minimize the package size.