Change CSS so the page is scrollable instead of the content div being scrollable

This was moved from https://github.com/oerpub/github-bookeditor/pull/146 (screenshots and comments)

This should make it a bit easier to:
- scroll while dragging a semantic block around
- add a top banner bar

In a subsequent PR I would like to:
- remove a bunch of the outstanding CSS cruft that is no longer needed (ie `height: inherit/100%` and `width: calc(100% - ...)`)
- remove the `###px` hacks and use LESS variables so it is clear how they are being generated