#5907810 Adds renew monthly option for annual plan downgrades

Changes proposed in this Pull Request

• Context: pcbrnV-1Zt-p2#comment-3259
• This change adds the “Renew Annually” and “Renew Monthly” links on the manage purchase page.
• Calculates the savings % for the annual plan and displays it next to “Renew Annually”.
• To be deployed only after D60535-code is shipped.

Testing instructions

• Purchase any annual plan.

• Navigate to /purchases/subscriptions/ and click on the purchased plan.

• Ensure Auto-renew is on and the plan expires < 30 days from today. (Use SA to change plan expiry)

• Refresh the page and confirm that the “Renew Annually” and “Renew Monthly” links are present.

• Ensure Auto-renew is off and the plan expires < 90 days from today.

• Refresh the page and confirm that the “Renew Annually” and “Renew Monthly” links are present.

• Confirm that the savings % is correctly calculated and shown next to the “Renew Annually” link.
  

• Clicking on “Renew Annually” should redirect to the checkout page with the renewal for the plan in the cart.

• Clicking on “Renew Monthly” should redirect to the checkout page with the monthly plan in the cart.

• To test the purchase flow for “Renew Monthly”, please see D60535-code