Split {views,serializers}.py in API into modules

Refactoring to work with new project models & attrs to follow.
Fix file names in /api/views
Start adapting API for new project/model relations

* Re-enabled API in settings.INSTALLED_APPS

* The base API view now sets request.project-- we could take a different
approach, but I think this makes sense.

* I added a new permissions class that gets the necessary permissions
from a view and then checks them against request.user.has_project_perms

* Added pre_save hooks for documents & notes to set objects' projects
when creating them (topics still to come)

* Reconfigured URLs