Onboarding Improvements: Reuse Post Sign-up Interstitial if there's no sites

[momo-ozawa]

Ref: #17520 (comment)

Description

This PR simplifies the Login flow for when there's no sites yet by reusing the Post Sign-up Interstitial screen.

Login logic for when there's no site yet:

• Before: Post Post Sign-up Interstitial screen shown on first login, and no sites Epilogue variation shown on subsequent logins
• After: Post Sign-up Interstitial screen shown on every login

Before	After

How to test

1. Log out / log in to a site w no sites
2. ✅ Make sure you see the Post Signup Interstitial screen
3. Repeat steps 1,2
4. ✅ Make sure you see the Post Signup Interstitial screen again

Regression Notes

1. Potential unintended areas of impact
   N/A

2. What I did to test those areas of impact (or what existing automated tests I relied on)
   N/A

3. What automated tests I added (or what prevented me from doing so)
   N/A

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

You can trigger an installable build for these changes by visiting CircleCI here.

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[leandroalonso]

Tested with WP and JP, no-sites and with sites.

[mokagio]

I'm going to merge this PR because it's been approved and has the 18.8 milestone, which is the code freeze I'll be starting today.