Background bulk add

Work in progress.

Still to do:
[ ] reinstate lost analytics event (cause of test failure)
[ ] delete batches after n days to avoid storing ephemeral data forever
[ ] sidekiq configuration in alphagov-deployment
[ ] check threadsafeness of papertrail and sidekiq (I suspect paper_trail isn't - if so we'll need to run one worker thread per process)
[ ] ensure errors from workers are reported to Errbit