This adds the required tracking for the new paid signup flow.
We're using frontend tracking as well because it gives us a user
session. For users who have not activated frontend tracking, we
can fallback to backend tracking.

Two things to be aware of:

1. Users who go to /-/subscriptions/new who are not signed in, get
redirected to /users/sign_up. After they successfully created an
account or signed in, we redirect them to /-/subscriptions/new again.
However, when they go to -/subscriptions/new, we would track the
experiment_start event a second time. Therefore, we added the param
experiment_started to not call track them twice

2. For the sign_up_view event, we only want to track users who got
redirected from /-/subscriptions/new, therefore we have added the
redirected_from=checkout param.