Purchases: Add analytics to the contact support links

[scruffian]

Adds a calypso_purchases_click_contact_support event to the contact support links on the cancel purchase page.

Fixes #271 (part of it)

Testing

1. git checkout fix/12207-analytics
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 cancel page
5. Click the "contact support" links (there are two)
6. Assert that you see this event: calypso_purchases_click_contact_support with the product_slug property.

• Code review
• QA review

[fabianapsimoes]

QA 👍

[drewblaisdell]

Do we need this line? purchase is a required prop, and this is only mounted when the purchase is loaded in CancelPurchase.

[drewblaisdell]

I updated your first commit to fix a small typo, and had one note about removing an unnecessary condition from trackClickContactSupport. If you're cool with that change, this LGTM.