Add initial models for teams, tweak projects

This adds an initial `accounts` app where we will manage users, teams and team membership. It should match our previous conceptual diagram, but it's open for review and discussion.

For the actual basic user sign up and management functionality (e.g. registration), I am planning to try out http://django-user-accounts.readthedocs.org/

For permissions and object ownership, I'll probably set up some special manager, following a similar scheme to what we use currently at work.