Admin UI improvements.

A bunch of usability updates for the admin panel. The exact changes are detailed in: https://github.com/18F/peace-corps-product-management/issues/202#issuecomment-66844322

Includes data migrations to point Featured Project Front Page objects at the account field instead of the pk-When switching over to raw_id fields, I wanted those raw_ids to be account codes, in response to feedback from the Peace Corps team that they think of projects in terms of their account codes.