Cleaned up models and views a bit.

Mediatypes for EPUBs are out of this branch, new content gets a
temporary id, and book editor can be configured for the default
mediaType for new content
Merge branch 'bookish-only' into epub-only
Override the mediaType for new content. Register EPUB mediaTypes
Merge branch 'epub-only' into dev
Merge branch 'dev' into gh-pages