move Zendesk ticket raising to background

before this change, tickets were raised in Zendesk synchronously.
This change:

* introduces 'sidekiq' (http://sidekiq.org/), a redis-backed
background worker queue
* moves raising the tickets to the background

The motivations for this change are:

* to respond snappier when the user submits a form
* to not lose user submissions when Zendesk is down
remove javascript associated with zendesk_error page
add endpoint for checking queue lengths
ZD exception test: functional => unit
sidekiq now uses existing redis.yml configuration