Change the way we apply bouncefix

First ever PR, hope I did this right!

**Status**: *Opened for visibility*
**Reviewers**: @tedtate @scalvert @kpeatt 
  
## Changes
- Previously, we were only adding bouncefix to `pinny__content` which made other absolute positioned scrollpanes within `pinny__content` unscrollable. (See: WHSmith tablet's sliding cards navigation)
- Now, we will add bouncefix to `pinny--is-scrollable` and we can add that modifier class to any scrollpanes within Pinny that needs scrolling.
- `pinny__content` by default will have the `pinny--is-scrollable` modifier class attached.
  
## Research resources
- https://github.com/jaridmargolin/bouncefix.js