FFS-2492: store applicant id in session, use it when logging events as the dist…

[millerti]

FFS-2492

Changes

Use applicant ID as distinct ID on Mixpanel

Testing Instructions

1. Log a few events
2. Log into Mixpanel and verify that the Distinct ID being used for your new events uses the Applicant ID

Acceptance testing

• No acceptance testing needed
  • This change will not affect the user experience (bugfix, dependency updates, etc.)
• Acceptance testing prior to merge
  • This change can be verified visually via screenshots attached below or by sending a link to a local development environment to the acceptance tester
  • Acceptance testing should be done by design for visual changes, product for behavior/logic changes, or both for changes that impact both.
• Acceptance testing after merge
  • This change is hard to test locally, so we'll test it in the demo environment (deployed automatically after merge.)
  • Make sure to notify the team once this PR is merged so we don't inadvertently deploy the unaccepted change to production. (e.g. :alert: Deploy block! @ffs-eng I just merged PR [#123] and will be doing acceptance testing in demo - please don't deploy until I'm finished!)

[tdooner]

How will this work if the vast majority of the mixpanel events don't have the cbv_applicant_id field?

[millerti]

Well, it wouldn't. Maybe I'm misunderstanding how the applicant id works. I assumed that all people going through the cbv flow would have one, invited or not. But if that's not the case then maybe I should fall back to a distinct ID of "invite-#" when applicant ID is missing.

[millerti]

I've verified all the events have the applicant id