Sidebar tree loads faster for large books

Node children are lazily parsed into ItemViews instead of (re)rendering
the whole tree and then hiding nodes.
Sidebar tree loads faster for large books
Creating new Folders works
Removed extra view redraws

caused some elements to show up twice
Merge branch 'bookish-only' into atc
Fixed "dirty bit" when saving

Enabling the "Save" button should work properly and unsaved content
should be visually marked as such.
Merge branch 'bookish-only' into atc
Reduced number of times views are rerendered
Pin bootstrap to specific version
Merge branch 'atc-pin-bootstrap' of https://github.com/philschatz/github-book into phil-atc-pin-bootstrap