Add an "add" button for creating new content
Refactored MEDIA_TYPES factory and BaseModel

 so Tree navigation and DnD is more uniform
Sidebar now shows Books and Folders that can be edited directly

Simplified the MEDIA_TYPES even further so it only contains a mapping
from mediaType to constructor.
Load content in the sidebar so it is navigable
Merged from 'bookish-only'
Added methods to parse/serialize folders