Refactor permissions

[nathan-osman]

The permission system currently works, but it is overly complex and consequently there is a lot of code repeated, violating the DRY principle. It would be nice to consolidate the permissions into a small group of methods on the Article model. For example:

• can_view() - determine whether the provided user can view the article
• can_edit() - determine whether the provided user can edit the article

These are boolean methods. However, we also need a @classmethod that returns a modified queryset that filters articles that the user cannot view. This is used basically anywhere a list of articles is needed/displayed.

This needs to be carefully planned and thought out to avoid the organizational problem that currently exists.