Use a 'singleton' field rather than the 'queue' field for singletons

@kbarrette - In order to split the queues into interactive and non-interactive variants we need to give the singleton queue indicator its own column, because there is a job that needs to be both a singleton and interactive (the store/MID provisioning job on grs_integration).

See https://trello.com/c/2lUi8y8B