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
Update padding for content in folders
Merge branch 'bookish-only' into epub-only
Updated Models to use the new MEDIA_TYPES factory
Merge branch 'epub-only' into dev
Return a default mediaType if none is matched
Remove an internal Node when dropped

instead of making a copy
Merge branch 'bookish-only' into epub-only
Support new MEDIA_TYPES factory
Merge branch 'epub-only' into dev
Support new MEDIA_TYPES factory
Load MathJax after everything else loaded