Use django-async-messages to send messages when a LabelEdit job is done
Add async messages for existing async jobs
Update background color of selected issue
Use real django messages on the label editor
Load messages on every requests, even ajax ones

This is done automagically:
  - a middleware add the messages to ajax template responses
  - a jquery ajax converters move the messages from the ajax response to
the document body
Manage unicode in AddMessagesToAjaxResponseMiddleware
Auto-hide messages after a delay