[WIP] Add /api/ endpoint

[toolness]

This attempts to add a Django REST framework-powered API at /api/.

To do:

• [ ] Consider exposing Client endpoint.
• [ ] Figure out how we want to expose the related Client for a Project in the /projects/ endpoint. For instance, do we want to embed the information inside each project, or do we want it to be a primary key that needs to be looked up at a /clients/{pk} endpoint?
• [ ] Figure out how we want to represent our model fields that have choices options. Currently, for example, Project.status is an integer that corresponds to tentative/active/paused/complete. But we might instead want this to be an actual string describing the choice.
• [ ] Figure out what fields we want to expose on Project. For example, do we want to expose mb_number? Alternatively, do we want to expose only a strict subset of fields by default, but show additional fields if they're specified as querystring arguments?
• [ ] Add CORS support for the API.

[brendansudol]

per discussion (over video), closing this for now until we have a specific api client and use case -- good call, @toolness!