Adapt note view to recent changes

1. New template tag for note sections that point to the appropriate
template for that section. By default it's a template in
"includes/note_sections/{section_type}.html", but that can be overridden
in the model.

2. Select note section subclasses in the note view.

3. Update templates, blah blah

4. Update migrations to assign projects to notes & documents to make
sure that the serialized data in their reversion.version objects were
reconstructed properly.