Move asset_migration queue to the lowest priority
Move asset_migration queue to the lowest priority

We're about to migrate about 1.5m attachments from Whitehall to Asset
Manager and we think this might take a few days (partly because our
Sidekiq config defines a concurrency of 1). This would block any
`link_checks` jobs (currently scheduled at about 2am each day) from
running until all the assets have migrated. Moving the asset_migration
queue to the bottom of the list will ensure that Sidekiq will only
process our jobs once the other queues are empty.

alphagov/whitehall,alphagov/whitehall,alphagov/whitehall,alphagov/whitehall