Purchases: Add analytics to the purchase confirm cancellation submit button

[scruffian]

Adds a calypso_purchases_submit_cancel_form event to the submit button form the cancel confirmation page.

Fixes #271 (part of it)

Testing

1. git checkout add/analytics-submit-cancel-confirmation
2. Open http://calypso.dev:3000/purchases
3. Add calypso:analytics to the debugger so you see analytics calls in your console
4. Open a purchase and go to the confirm cancel page
5. Submit the cancel button
6. Assert that you see this event: calypso_purchases_submit_cancel_form with the product_slug property

• Code review
• QA review

[fabianapsimoes]

QA 👍

[drewblaisdell]

Super minor, but I believe this should be calypso_purchases_cancel_form_submit per the analytics readme:

In order to keep similar events grouped together when output in an alphabetized list (as is typical with ananlytics tools), put the verb at the end of the event name

[drewblaisdell]

I updated your commit to use getPurchase instead of accessing this.props.selectedPurchase.data (this seems to be a pattern in components that use PurchasesMixin) and added a commit to rename the event name, placing the verb at the end. Feel free to squash that if you agree with the change.

LGTM.