Move bouncefix.add('pinny__content') into pinny open to fix scroll jank on some sites

Status: **Opened for visibility**
Reviewers: **@kpeatt @scalvert**

## Changes
- Move the initialization of bouncefix into pinny.open
- Removes bouncefix when pinny is closed

## Notes
- On some sites it seems like pinny can introduce some pretty bad scroll jank and the cause of the jank comes from bouncefix. This change fixes that issue.
- It would be nice if we could figure out in what situations the jank is introduced. Because it does not seem like bouncefix has this effect in all situations.