#7884827 Add user’s email address to the Subscription Management portal

Fixes #74822.

The proposed changes add user’s email address to the subheading of the Subscription Management portal for non-WPCOM users:

Proposed Changes

• create helper getSubkey function
• introduce useEmailAddress hook that extracts user’s email address from the subkey cookie
• introduce useSubHeaderText hook that outputs the subheading text based on whether the email address is available

Testing Instructions

Checkout the branch related to this PR and build Calypso locally.

Non-WPCOM user

1. Trigger the “subscription management link” email by submitting the form on https://wordpress.com/email-subscriptions/.
2. Open the link in an Incognito browser window:
   
3. Open the browser inspector and copy the subkey cookie value as it is (without decoding it):
   
4. Navigate to http://calypso.localhost:3000/subscriptions/settings.
5. Open console and set the cookie by document.cookie = "subkey=PASTED_COOKIE_VALUE";.
6. Reload http://calypso.localhost:3000/subscriptions/settings → the correct email address should display in the subheading.

WPCOM logged-in user

If you navigate to http://calypso.localhost:3000/subscriptions/settings as a logged-in WPCOM user, the subkey cookie is not available and hence the subheading will be as follows: “Manage your WordPress.com newsletter and blog subscriptions. (the original subheading copy)”:

Pre-merge Checklist

• Has the general commit checklist been followed? (PCYsg-hS-p2)
• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• Have we added the “[Status] String Freeze” label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
• For changes affecting Jetpack: Have we added the “[Status] Needs Privacy Updates” label if this pull request changes what data or activity we track or use (p4TIVU-ajp-p2)?