Localisation using i18next

Alternative for !41 (closed) closing #22.

• Makes sure there is just one Moment.locale call setting the device locale.
• Send the device language to the server for the correct localised responses.
• Adds yarn locales:collect command to automatically find and update all locales files.

Advantages over !41 (closed):

• Made for React, using react-i18next
• Autocollect all translations
• Uses no abstract keys, the English base translation ís the key. Thus making translations easier to read and support.