Merge pull request #57 from wluo/flock

also fire the 'go_subscriptions_created_guest_user' when an existing user signs up
Merge pull request #58 from wluo/flock

detect if a signed-in user comes to the sign-up page via the corporate purchase page
Merge pull request #59 from wluo/flock

catch few more cases where the user tries to subscribe to a plan that's at or lower than the user's current subscription level
also populate the user's email on individual signup page if we have it. styled the read-only email to be slightly lighter. see https://github.com/GigaOM/legacy-pro/issues/4550