Bugfix: Scrolling inside Markdown Editor

Scrolling inside the Markdown Editor was broken, before the redesign by 
allowing the user to scroll the appBar out of the viewport, and after 
the redesign by cutting a potentially long preview off at the bottom.
This has been addressed by allowing to scroll the preview content.

https://github.com/owncloud/web/issues/4606
https://github.com/owncloud/web/pull/6386
