Part of #11705

To test

Full Flow

0. Clear app data.
1. On the Prologue Screen, tap Log In.
2. If the Smart Lock dialog appears, dismiss it.
3. On the Email Screen, enter an email address that is not associated with a WordPress.com account.
4. Tap Next.
5. Notice the Sign-Up Magic Link screen.
6. Check email inbox of address used in Step 3.
7. Notice signup email from WordPress.com.
8. Complete the sign-up flow normally.

Jetpack Flow

0. Log in with a self-hosted site that is not associated with a WordPress.com account.
1. Go to My Site tab and tap Stats (or go to Notifications tab).
2. Tap the Install Jetpack button.
3. If the Smart Lock dialog appears, dismiss it.
4. On the Email Screen, notice there's no Don't have an account? Sign up button.
5. Enter an email address that is not associated with a WordPress.com account.
6. Tap Next.
7. Notice the Sign-Up Magic Link screen.
8. Check email inbox of address used in Step 3.
9. Notice signup email from WordPress.com.
10. Complete the sign-up flow normally.

Notes

• This doesn't add the Sign-Up From Login functionality for the Google Sign-In method just yet. This will be done in the following PR.
• This also doesn't remove the Sign-Up button from the Prologue screen. This will be addressed in a future task.
• Lastly, this also doesn't change the current labels used by the flows, so it may not always make sense.
• This PR make changes to the LoginFlow library. These changes will probably need to be merged to the LoginFlow library once our feature branch gets merged back to develop.

PR submission checklist:

• I have considered adding unit tests where possible.
• 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.