Stepper: improve calypso_signup_start tracking

[daledupreez]

Details

As discussed in a call earlier this week (notes are in p58i-hhJ-p2), we are seeing the calypso_signup_start event being triggered too many times for flows built using Stepper.

The core issue is that we may trigger a calypso_signup_start event when we first load the flow, but we then redirect to /start/account/user-social to get the user signed up, and we then trigger a second calypso_signup_start event when we're redirected back to the flow again.

Related to:

• Stepper: only trigger calypso_signup_step_start Tracks events when step is actually rendered #91243
• Improve login redirects and step tracking in stepper #91241

Checklist

• When we start a Stepper flow flagged with isSignupFlow: true, we should only create one calypso_signup_start event, even when we redirect the user to a separate user signup step.
  • This will require a clear definition of when we should trigger the event, and when we should not trigger the event.