Stepper: mark pending actions

[escapemanuele]

Right now we trigger calypso_signup_complete event after completing a processing action, in the processing step.

For extra safety, and to be able in the future to support multiple pending actions, we can enrich the pending actions with a "is signup complete candidate" prop - meaning, not every pending action should immediately become the condition for recording the signup complete event. Assuming these pending actions have any other value in the stack.