Add monthly pricing test #5048137

Changes proposed in this Pull Request

• Add the monthly pricing test to Calypso. This PR requires the server-side implementations.

Testing instructions

💡 For full and highly-detailed test instructions, please see: pau2Xa-2ak-p2

— or —

• Make sure you’re proxied.
• Sandbox public-api.wordpress.com which D51146-code is applied to.
• (added) D51640-code also needs to be placed on the sandbox.
• Assign yourself to a test group of monthly_pricing_test_phase_1 by following the instruction ( pau2Xa-251-p2#comment-7999 )
• Assigny yourself to the treatment group of monthlyPricing a/b test.
• Start the process of creating a new website with the onboarding flow( /start/user ).
• If you’re in the treatment test group, the plans page should show monthly dotcom plans as below:
  
• Pick a plan except for the free plan.
• Your site is newly created and you will see the checkout page that shows what you get with annual/biennial plans.