Allow configuration of which workflow to depend on (per field)

I considered making the config storage (third-party settings) support multiple workflows, but treat it as a single value on the front-end. The idea being that it'd be easier to move to supporting multiple workflows later, as the data structure would already be in place. However, this approach only seemed to incur more complication that it was worth.

If ever we need to support multiple workflows, we can start a 2.x branch, and handle re-writing configs in a hook_update().

Automatically closed - issue fixed for 2 weeks with no activity.