Replace the schema with a full dtbook schema

The full dtbook is extended with elements to handle braille. This
enables upload of normal dtbook files to Daisy Producer.
Implement the Open Document preview

- add a backend to the pipeline cli in external.py
- add a view in todo.py
- add some templating code
- add a url to the view in urls.py
Add some minimal form of error handling for the odt preview
Add the brl prefix to all brl elements
Parse errors from stdout of the pipeline cli
Add support for images

- Add an image model that relates to a document and contains a FileField
- Add an corresponding migration
- Use the images when building the zip to invoke the pipeline2
- Add image upload to the details page for documents
- Add some weird hacks to make sure that when uploading the same image
  doesn't create a new db record but instead just replaces the
  existing record and file. For that a storage is implemented and the
  save method of the model is overwritten
Add an admin view for Images
There is no need to remove the file in the storage.

Just return the name as is and this will ensure that the existing file
will be overwritten.
Allow track:Guidelines and track:Supplier in the meta data

as the xml provided by supplier contains it
Add a FIXME comment to the save method of the Image model

as it turns out that the save method does about 10 SQL statements to
do its work.
Handle non-valid Image upload
Remove prototype.js
Add the jquery file upload plugin

so the user can upload many images using drag and drop
Add more allowed languages for documents
Limit the shown images to 7
Enable translation of image elision
Make sure the dom is properly updated when uploading images

Especially the "N more..." part
Enable spanish documents