Add basic Sidekiq worker for imports

This works as-is.
Add named queue for imports, and config

Having explicit configuration means we can avoid naming all the queues
on the command line.
Move to sidekiq by pulling out ImportRowWorker

Also moved ProgressLogger out of Import so that it can be used by the
ImportRowWorker.

Removed stubbing from Import unit tests to simplify them. It makes them very
marginally slower but we feel the benefit of simpler tests outweighs the
speed.
Fix hash syntax.
Run all Sidekiq tasks sequentially in all tests.
Fixup import tests inc database truncation.
Remove some redundant methods & calls.
Whitespace.
Tidy up ImportWorker.
Add Foreman worker to run Sidekiq

This is used by Upstart on the non-dev environments.
Kill ImportWorker#error.

Left over from delayed_job.
Merge pull request #893 from alphagov/sidekiq-background-workers

Tracker: https://www.pivotaltracker.com/story/show/57390266

Re-implement importer using sidekiq