Upgrading celery and dependent libraries (bug 825938)

Note: Don't merge until the backend is ready to upgrade rabbitmq.
Bumped billiard for celery upgrade
Polish translation
Switch Persona URLs (bug 872168)

This also enables experimental prefixes (bug 850084)
Merge pull request #122 from Witia1/master

Polish translation done
Merge pull request #123 from kumar303/persona-url

Switch Persona URLs (bug 872168)
Fix verifier experimental params (bug 850084)
Log the persona logout
Log redirects to debug @enforce_sequence
Make confirm PIN more sticky (bug 872299)
Merge pull request #124 from kumar303/reset-pin

Make confirm PIN more sticky (bug 872299)
Progress indicators (bug 861133)
Merge pull request #115 from muffinresearch/progress-indicators-861133

Progress indicators (bug 861133)