Allows the language selection menu to be active if `?enable=language` is in the URL, otherwise shows a ‘coming soon’ tooltip. This can also land on a branch if we don’t want that available on preview.

- language menu opens if the globe is clicked
- language menu closes if mouse is clicked anywhere outside the language selector
- language list is imported from scratch-l10n
- current locale is updated if another language is selected (i.e. the current language selected changes)

Not in this PR:
- styling of language selector
- actually changing the messages!

Allow intl to be passed in to GUI as a prop (rather than having to be already in the store)

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