* Added script to create combined locales `Blockly.ScratchMsgs.locales`
* Regenerated en.json after changing single quoted strings in messages.js to double quoted strings.
* Include `create_scratch_msgs.js` in `npm run translate`
locale=<locale-string>` to the URL.
* Added assertions so translate fails if `messages.js` has incorrect strings, or if any of the downloaded locale json files have errors in the translations.
* Playground will load different locales.
