This will make the unread messages feature work.
Update so that new chats are opened in the background.

Ensures that max one chat box is visible at any given time.
For use in mobile or fullscreen applications.

with lodash and Backbone.

Looks like now that we use Backbone.Events we don't get event objects passed
to the handlers.

Refactor locales code to store the currently chosen locale as
`_converse.locale` and use that in converse-notifications.js

- Fix event listener code examples
- Mention the preferability of using versions in the CDN

Only $.Deferred is still being used in converse-core.