Disables the Jetpack No Sites error for logins and signups

[emilylaguna]

Since we're allowing site creation we shouldn't display the no sites error if the user logs in or signs up.

This also fixes a small issue where the tapping on the 'Go to Reader' button from the Notifications.

To test:

1. Follow the steps from here: Enable more features in the Jetpack app #17914

No Sites Login

1. Open the app
2. Tap the Continue with WP.com button
3. Login to an account that does not have any linked sites
4. Verify you are brought to the rest of the login flow and not displayed an error

Signup

1. Open the app
2. Tap the Continue with WP.com button
3. Create a user account
4. Verify you are brought to the rest of the onboarding flow and not displayed an error

Regression Notes

1. Potential unintended areas of impact
   None

2. What I did to test those areas of impact (or what existing automated tests I relied on)
   None

3. What automated tests I added (or what prevented me from doing so)
   None

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.

[emilylaguna]

cc @ashiagr

This PR disables the no sites error message for Jetpack app now, thanks for bringing this up!

[wpmobilebot]

You can test the Jetpack changes on this Pull Request by downloading it from AppCenter here with build number: pr17948-960c538. IPA is available here. If you need access to this, you can ask a maintainer to add you.

[wpmobilebot]

You can test the WordPress changes on this Pull Request by downloading it from AppCenter here with build number: pr17948-960c538. IPA is available here. If you need access to this, you can ask a maintainer to add you.

[momo-ozawa]

Works as described.

One small thing. I noticed the asset we use in the PostSignupInterstitialVC is blue instead of green. Maybe we can reuse jetpack-empty-state-illustration.

[momo-ozawa]

@emilylaguna One other thing -- we should update the release notes. (Thanks for the reminder @ashiagr!)

[emilylaguna]

@momo-ozawa I updated the image, and the release notes. Can I get another review please?

[momo-ozawa]

The image looks good! Left 1 minor suggestion.