restructure folder model to match book

- changing title of content changes it on the content (no override
titles)
- changing title marks folder as dirty
exclude "New Chapter" from the book picker add button
update the new URL for book and folder
clear dirty bit on folder save
make a pointer when dropping to folder
only open content view if the pointer points to something with one
css tweaks to remove scrolling
add folder styling to sidebar
Merge branch 'github' into atc-update