Plans: update grid for introductory offers

[michaeldcain]

This updates the plans grid to properly account for different introductory offers for WPcom plans. It updates the Introductory Offer plan descriptions and consolidates some of the display logic between discounts and introductory offers.

Requires: D134559-code See: pbOQVh-4sd-p2

Before	After

To test:

• enable Store Sandbox and sandbox public-api.wordpress.com
• create a new user, or use a user that has not previously subscribed to one of these plans
• change your user currency to MXN, PHP, or INR
• create a new site and verify that the different plans grids correctly reflect the introductory offer pricing (reduced monthly breakdown, intro offer full price in the description, and correct billing term in the description)