Don't clear the email input when user returns to it by clicking `Go back`

### What
While it's expected that the app will autologin once the user completes the sign-up process, it will not hurt to keep the entered email address when the user clicks `Go back` in the `we've sent an email` prompt:

https://user-images.githubusercontent.com/73365754/114924268-78f9ee00-9e36-11eb-9f76-acb77107a890.mov

### Why
If the user completes the sign-up on a different device, it's friendlier not to make the user re-type the email once again.

Reproduce the sign-up flow through the “we've sent an email” prompt, then click “Go back” as shown in the linked video. Trace the email input's state across that transition and verify that the entered address is still present when the user returns to it.

Written by the indexing model from the issue text.