fix(content): Email from subscriptions redirect not passed to react

[vpomerleau]

Because

• Email address is lost when accessing the “here” link from “Automatic renewal notice” email with the force experiment parameter added

This pull request

• Set the email on emailFromIndex instead of email to ensure it is passed to React force_auth/signin

Issue that this pull request solves

Closes: #FXA-9975

Checklist

Put an x in the boxes that apply

• [x] My commit is GPG signed.
• [ ] If applicable, I have modified or added tests which pass locally.
• [ ] I have added necessary documentation (if appropriate).
• [ ] I have verified that my changes render correctly in RTL (if appropriate).

Screenshots (Optional)

Please attach the screenshots of the changes made in case of change in user interface.

Other information (Optional)

If a renewal notice is not available for testing, these steps may be an adequate scenario for testing:

Pre-requisites

• Have an account with an active subscription (e.g., added from 123done);
• Sign out of account and clear local storage (localhost:30330/clear)

Steps

• Go to localhost:3030/subscriptions with React experiment params and account email as param e.g. with an account created for test@gmail.com: http://localhost:3030/subscriptions?forceExperiment=generalizedReactApp&forceExperimentGroup=react&email=test%40gmail.com

Expected result

With this PR: redirected to signin with password (prompt to enter password for test@gmail.com), then redirected to subscriptions after sign in

Before this PR: redirected to index page (prompt to enter email), then redirected to subscriptions after sign in