create a Proposal model

:warning: **WIP** :warning: 

I have been thinking about [how to generalize](https://github.com/18F/C2/issues/109) the app, and short of actually splitting the application into multiple pieces, this PR splits a Proposal model out from Cart. The medium-term goal is to separate any GSA Advantage-specific logic from that dealing with the overall workflow and approvals. After this, I plan to create [a new API](https://www.pivotaltracker.com/story/show/77296838) for Proposals (not Carts), which can be used by Navigator, WHSC, etc.