#7504755 Emails: add visual indicator about save with annual email subscription

Proposed Changes

At the current moment, we don’t have visual tips for customers about the advantages of purchasing annual email plans.
So we decided to use a similar approach to the Selecting Plans page – we will show a popover in case a customer has selected a tab with a monthly subscription and will hide it if the selected tab is for annual subscription.
NB: Textual information will be discussed and changed before merging this PR.

Testing Instructions

• Go to your dashboard
• Click on the “Switch Site” button in the top left corner
• If you have a site w/o email subscription:
  • Choose your site w/o an email subscription
• Otherwise, if you don’t have existed account w/o an email subscription – create it following the steps under this item:
  • Click on “Add new site” at the end of the appeared list
  • Enter any domain name for test purposes
  • In the appeared list below – choose a domain with the help of the “Select” button
  • Choose any plan from the proposed with the help of the “Select” button
  • Scroll down and pay for the subscription with the help of the “Pay {amount} with credits” button
  • Note, on the page “Add Professional Email” – click on the “Skip for now” button
  • On the page “What are your goals?” – click on “Skip to dashboard” in the top right corner
• Move the mouse on the “Upgrades” item in the left sidebar
• In the dropdown, click on the “Emails” item
  Expected Result: Pay annually tab is selected by default, and nothing changed. We see the next screen:

• Click on the Pay monthly tab
  Expected Result: Pay monthly tab is selected and appeared popover with textual information near the Pay annually button.

BEFORE	AFTER
	Popover is visible if selected `Pay monthly` tab

• Click on the Pay annually tab
  Expected Result: popover disappeared.
• Click again on the Pay monthly tab
• Let’s check responsiveness – if the screen width is less than 1280px, then popover is located under the Pay annually button:

Pre-merge Checklist

• 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)?

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