Onboarding/slack id entry

[getheobald]

Changes

Current routing:

• Sign up, guest, /home shows IntroGuestHomePage
• Click join NER and select subteam, /home shows /home/pnm (PNMHomePage)
• Finish recruiting dashboard, goes to /home/onboarding
• Click finish, history.push(routes.HOME_ACCEPT), AcceptedPage
• Click Accept, completedOnboarding now true, redirects to /home, renders GuestHomePage since still guest role

Changes I made:

• Have handleConfirmModal on onboarding page call completeOnboarding() and redirect to slack id page
• Get rid of AcceptedPage entirely
• Add persistent gate in AppAuthenticated - nvm, this already exists (SetUserPreferences)
  • Change if to (isGuest(userRole) && !completedOnboarding)
  • Pass in completedOnboarding from AppPublic
• New backend validation endpoint because rn SetUserPreferences doesn't do any validation

Notes

How important is it for me to tag commits with ticket number? I keep forgetting

Screenshots

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If you did any manual testing (e.g., with Postman), put screenshots of the http request and before and after of the db

If none of this applies, you can delete this section

To Do

Need to figure out if window.location.reload() is necessary

Checklist

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Please review the contributor guide and reach out to your Tech Lead if anything is unclear.
Please request reviewers and ping on slack only after you've gone through this whole checklist.

• All commits are tagged with the ticket number
• No linting errors / newline at end of file warnings
• All code follows repository-configured prettier formatting
• No merge conflicts
• All checks passing
• Screenshots of UI changes (see Screenshots section)
• Remove any non-applicable sections of this template
• Assign the PR to yourself
• No yarn.lock changes (unless dependencies have changed)
• Request reviewers & ping on Slack
• PR is linked to the ticket (fill in the closes line below)

Closes #4137