Plans: Add Tracks Events for Google Vouchers

[Essayoh]

Can we add the following Tracks events for the new Google Vouchers flow?

Customer clicks 'Generate Code' - https://wpcalypso.wordpress.com/plans/my-plan/ - proposed Event name: calypso_plans_google_voucher_generate_click

Customer clicks 'Agree' on Google Terms - proposed Event name: calypso_plans_google_voucher_toc_accept_click

Customer clicks 'Setup Google Adwords' external link button - proposed Event name: `calypso_plans_google_voucher_setup_click"