Frontend to allow changing email during signup

[nc225]

Implements the front end changes to close #9092. Backend changes are in this separate PR. This PR choose option 3 from this comment and lets users change their email at the email verification step.

On the design: I considered having the button to submit a new email address to the right of the text box instead of below it, but it seems like we usually put buttons below. I also considered having this all in a drop down accordion menu, but that didn't feel right for a single function. Very open to changing anything here, though.

Testing

(Note: Github's auto-checks on this repo will likely fail until the BE in #9627 is merged.)
I locally merged and ran both the frontend and backend branches, following all the steps in #9627 until "To run the backend tests..."

I went to localhost:3000 and tried a few different scenarios to break it, but everything worked as expected. I also added a few new frontend tests to check various scenarios and ran those frontend tests using yarn test.

Backend checklist

• Added tests for any new code or added a regression test if fixing a bug
• Run the backend locally and it works
• Added migrations if there are any database changes, rebased onto develop if necessary for linear migration history

Web frontend checklist

• There are no console warnings when running the app
• Added tests where relevant
• Clicked around my changes running locally and it works
• Checked Desktop, Mobile and Tablet screen sizes

For maintainers

• Maintainers can push commits to my branch
• Maintainers can merge this PR for me

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Other

Protos