Deactivation: add tracking for deactivations

When a customer decides to stop using Jetpack they most often do not properly disconnect and fill out our survey. They usually simply deactivate the plugin and presumably delete it.

When a customer clicks 'deactivate' in the plugin page, let's add tracking to that link to get a sense of how often disconnections are the result of a deliberate user action.

cc @beaulebens @johnHackworth